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roscoe6977
08-25-2006, 05:12 AM
I'm setting up an aquarium and using a canister filter for the first time. I normally try to follow a general rule that you should try to totally filter your tank 5-10 times per hour (250-500 gallons per hour for a 50 gallon tank). I know that closer to the 10 times is better for cichlids. My questions (and hopefully they aren't too foolish) are: Can you overfilter a tank? Do canister filters function at a higher lever than other types of filters? Basically if your canister is rated at 315 gph is that all you figure it is good for or based on the multiple layers of filter media, does the canister actually provide a higher level of filtration?

I have a Via Aqua 750 (rated at 315 gph) that I will be using on a 75 gallon tank. Also planning on 2 #4 sponge filters. Will I need a back of tank also?

cich1
08-25-2006, 10:37 AM
personally i ran a mid size cannister(rena filstar xp2) and an AC110 on my 75 when i had it running. the tank was always well filtered in my opinion. i don't think you can overfilter a tank, but you can have too much current in your tank and blow your fish all over hell.:D usually when they rate cannister filters for flow it is without any media at 0 head height, so you figure you are going to get a nice drop in pressure from the distance the water has to travel up to the tank, and after having to flow through whatever media you have in there. my vote goes to the cannister H.O.B. combo, for a 75.

gaines
12-13-2006, 04:41 AM
Originally posted by cich1
usually when they rate cannister filters for flow it is without any media at 0 head height, so you figure you are going to get a nice drop in pressure from the distance the water has to travel up to the tank, and after having to flow through whatever media you have in there. my vote goes to the cannister H.O.B. combo, for a 75.

What you're saying is not accurate. Yes, what you're referring to about head height applies when a pump has to move water from one level up to another. It does not apply when you are moving water from one level, through the canister, back to the same level.

The amount of pressure that the water is providing in the intake tube is the same exact amount of pressure that the water is resisting in the output tube. The net result is that you can hang your canister on the back of the tank or put it in the basement and it's height will have no bearing on the rate of flow.

As for the original question, it completely depends on how heavily you plan on stocking the tank, which also depends very heavily on what exactly you're putting in the tank. I ran a heavily planted 135 gallon on a Rena Filstar XP3 (350gph rated) successfully for quite a long time, but I kept relatively few fish in it and the plants helped control waste.

jbscichlid
12-14-2006, 12:55 PM
The net result is that you can hang your canister on the back of the tank or put it in the basement and it's height will have no bearing on the rate of flow.

This is not ture ethier, ALL canisters have a limit on the distance they can push water, and with any water pump the more highier they push water the more the flow rate goes down. For example; if I was to setup my Xp3 20 feet below the tank there is NO way the water would ever make it back to the tank. Now you are right in the fact that canister a disinged to run below the tank and ment to push water up and not suck water up, but every pump has it limits.

Canisters main advantages are quitness, media volume, and clean look, (nothing between the tank and the wall). Out of that canister with the media in you are probably geting any where between 130-180gph actual flow. The rated flow is based on the filter without media, I always assumed with the head height of the tubes that come wit the filter, but I could be wrong on that one. :)

RustyNut
12-14-2006, 02:02 PM
Guys Gaines was right.... in a closed system like a canister, the siphon effect at one end essentially nullifies the head height issue. Maximum head height won't change, but losses due to head height are only troublesome on open systems like a sump.

It is true that you lose flow rate with media in the filter, but ALL filter are rated without media, including HOBs.... In cannisters something called dwell time is important and mostly negates the effects of the lower flowrates. (at the cost of mechanical filtration)

Jeto
12-14-2006, 06:01 PM
Originally posted by RustyNut
Guys Gaines was right.... in a closed system like a canister, the siphon effect at one end essentially nullifies the head height issue. Maximum head height won't change, but losses due to head height are only troublesome on open systems like a sump.

It is true that you lose flow rate with media in the filter, but ALL filter are rated without media, including HOBs.... In cannisters something called dwell time is important and mostly negates the effects of the lower flowrates. (at the cost of mechanical filtration)

learned something new again....

RustyNut
12-14-2006, 08:28 PM
:) Never stop learning :wink:

Just think of a canister like a hose filled with water and both ends kept under the surface.... no matter how you orient the loop outside the tank, the force inside the hose is neutral(ized). Prick a hole (open) in that loop however, and the forces no longer oppose each other.

DogWalker
12-15-2006, 05:17 PM
Originally posted by RustyNut
Guys Gaines was right.... in a closed system like a canister, the siphon effect at one end essentially nullifies the head height issue. Maximum head height won't change, but losses due to head height are only troublesome on open systems like a sump.


This is actually quite a complex question once you scratch the surface. While the above argument is logical, it got me asking myself "why does the owner's manual for my old Fluval contain a chart showing decreasing efficiency the further below waterline the cannister is placed?"

I believe the answer to that question lies in the media: think of a solid body falling through air - it eventually reaches a terminal vellocity (beyond which it won't accelerate) due to resistance. The same effect will happen with water flowing through solid media, only much quicker. In short, when you increase the distance between the water line and the cannister, the pressure of the water entering the impeller chamber increases, but not as quickly as the pressure required to push the water out of the impeller chamber and back up into the tank.

There would be mitigating factors in play here: dwell time, as you mentionned will somewhat compensate for reduced flow in terms of real efficiency. But also, if there is any significant media bypass in the filter design this will mitigate any perceived reduced flow rate, but at the cost of more water bypassing the media. To throw another wrench into the problem not all filter motor characteristics are the same, and it is possible (don't know, haven't checked/tested) that my old Fluval may have been more affected by height than a different filter model (the old horsepower vs torque thing).

The only way to be really sure is to get an answer from the manufacturer of a specific filter model AND do your own tests -
these would be tricky: measuring any difference in flow rate would be easy enough, but measuring any difference in real filtering efficiency (taking dwell time, flow rate and media bypass into consideration) would be a lot more trouble than I'm willing to go to.

Cheers :)

Glaive
12-15-2006, 05:48 PM
I use extra vent filters to help reduce the pass through of my fluval cannister. I prefer a lower flow as that allows the bacteria more contact there by being more efficient. My main concern is always biological filtration as I can vacuum particles. As it stands my tank is almost always crystal clear. When I stir up my sand it usually takes 30 minutes to get there again.

On the cannister height to tank height issue, no matter how efiiciently a system is designed perfection is impossible.

I agree with Lojack here. A cannister is not a perfectly closed system. A cannister that delivers 100 gph at a head height of 2 feet will not deliver 100 gph at 10 feet. If I had a flow rate meter that was stainless steel I would test tank base height vs 1 foot below in a real world environment.

RustyNut
12-15-2006, 10:12 PM
Guys, that was why I said maximum head height remains... the pump can only push uphill so far....

also this isn't a zero sum loss.... there are still plumbing restrictions and length of tubing to consider as well as there is some loss from height... I only stated that most is being countered by the siphon effect.

gaines
12-15-2006, 10:17 PM
I stand by my original statement that there is no maximum head height in a closed system such as a canister filter. I could theoretically run a tube hundreds of feet down from a tank into a tiny little pump and hundreds of feet back up. Even with a tiny pump it will still be able to move the water because although it's having to elevate water hundreds of feet (under a lot of pressure) it is also helped by water pushing down an equal amount hundreds of feet in the intake tube.


Note that in a closed loop system, the static head is zero because the fluid on one side of the system pushes the fluid up the other side of the system, so the pump does not need to overcome any elevation.

In closed systems the starting point is the same as the ending point. Therefore the beginning velocity equals the final velocity, so velocity head is not a consideration.
http://www.fluidh.com/WebCalcPumpHead.htm

The only caveat I will add is that the more tubing you add to the system the more resistence from friction results. It's common practice in science to ignore factors such as friction when they do not have a significant effect on the scenario. In your average canister system there is so little tubing that the amount of friction created by the water moving against the inside of the tube is negligable.

DogWalker
12-16-2006, 12:07 AM
For the sake of argument let's say we ignore the flow resistance of the tubing. The force of the water pushing down is reduced by the resistance of the media it must pass through before reaching the pump, so it is not equal, in practise, to the force required to get the water back to the tank. This effect will be more dramatic the further below waterline the cannister is because the opposing forces will be that much greater. The greater these forces are the more important it is for them to be in perfect balance. If you put your cannister 100 ft below your tank the opposing forces will be so great that the downward force will only need to be reduced by a small percentage for the pump to become ineffective.

I would be inclined to agree with your view if there were no media in the filter.

Anyway, to get back to the OP's question, a cannister/hob combo would be good on a 75 for cichlids. The above discussion aside, I believe most of us calculate our filtration based on the manufacturer's stated flow rates, and accept that in a typical setup on a typical height stand with a typical load of media the flow rate will be less. Providing your filtration eliminates amonia and provides whatever circulation you need it to provide then it's all just down to you doing your regular water changes.

gaines
12-16-2006, 12:40 AM
Originally posted by DogWalker
For the sake of argument let's say we ignore the flow resistance of the tubing. The force of the water pushing down is reduced by the resistance of the media it must pass through before reaching the pump
I'll stop you right there, because the force of the water at 100 feet (before entering a canister) is a result of the weight of the water. We're talking about gravity which is a constant (well, ok, it's not quite a constant but close enough). This is the force that cancels out the opposing force on the other side of the canister filter. If those two forces cancel each other out, then we're now talking about moving the water from one side of the canister filter to the other. Yes, the more media you have in the filter the lower your flow, but this does not change based on the altitude of the canister.

If you don't believe me, look at the link in my previous post. That's a very in-depth description of how this all works written by a company that specializes in water pumps.

RustyNut
12-16-2006, 06:55 AM
Originally posted by DogWalker
I would be inclined to agree with your view if there were no media in the filter.

Media plays no role.... the media is simply resistance.... no different than the tubing restrictions. The media isn't a force, so it doesn't "counter" anything. It resists the waters passage, so the more media you add the lower the flow through the restriction (friction), but can't stop it.
This is all Physics 101 stuff.... Forces are added together (positive and negative) and then the results are reduced by frictional losses... we don't apply frictional losses directly into the force side of the equation, but only after we have calculated all the forces together.
Remember that friction is high initially, but drops once movement begins. (It might take a force of 150hp to get the truck moving to a given speed, but it only takes 30hp to keep it moving at the same speed.) This is why frictional losses are calculated after forces.... they are variable and sometimes difficult to quantify.

DogWalker
12-16-2006, 07:46 AM
Gaines, that is a good link, you just need to properly apply the information therein to this particular application. Too bad they don't expand further on the subject of friction head.

Rusty, the 'physics 101' explanation you've offered has always been obvious to me. It is the right starting point to understanding this problem, but not the whole answer imo. This is not a truly closed system in that the friction caused by the media affects the income water, but not the water returned to the tank. Your tank is the "break in the piping system that interrupts forced flow" that makes this an open system. The proof: remove the spraybar from the tank with the filter running - the intake and output are independent and will continue functionning. until the water level drops below the intake. Keep adding water to the tank during this test and it will go on and on.

"For open systems: Pump head = the sum of all friction losses plus the static lift of the fluid plus the pressure head."

By seeing this as an open system it is easier to understand that the friction loss on the intake is greater than that on the output, due to the media. The next question is why would height make a difference - isn't the friction loss a constant? No - it isn't a constant. Higher water pressure (from greater height) would if unrestricted result in higher velocity. The higher the velocity the higher the resistance since the media has nowhere to go. Since you don't have this added resistance on the return, greater height = greater net difference between the force of the water entering the impeller chamber and the force required to push it back up to the tank.

gaines
12-16-2006, 09:25 AM
Originally posted by DogWalker
Gaines, that is a good link, you just need to properly apply the information therein to this particular application. Too bad they don't expand further on the subject of friction head.

What don't they explain about friction head? I could include a number of links to engineering sites that all explain friction head the same way, but there's not much point as they all come up in Google if you type "friction head".


Originally posted by DogWalker

remove the spraybar from the tank with the filter running - the intake and output are independent and will continue functionning. until the water level drops below the intake. Keep adding water to the tank during this test and it will go on and on.


You can't change the definition of something by showing that you can change it to meet that definition. Your body's circulation system is a closed system. That is a widely accepted fact. I can't argue that by cutting an artery and making your circulatory system "open" I've now shown that circulatory systems are open systems. Just because you can change a canister filter to spray into another body of water effectively making it an open system doesn't mean it was open in it's original design.

RustyNut seems to have a good handle on the situation.

Glaive
12-16-2006, 10:25 AM
Okay reading the link and having rusty refresh my memory makes this all make sense.

With out friction a cannister will maintain a specific flow no matter the length of the system. In reality there is a loss in flow due to friction within the system. So the reason for a lower flow over a longer system is not head height but the friction of the system. It makes sense.

RustyNut
12-16-2006, 10:27 AM
Originally posted by gaines
Just because you can change a canister filter to spray into another body of water effectively making it an open system doesn't mean it was open in it's original design.


That is exactly it! Once you remove the output to another tank (whatever) the system is no longer closed.

I think we both mis-spoke here... friction IS a constant, resistance is what changes in relation to velocity. - let's not get our terminology mixed up. We've already agreed with you that media is a resistive force that decreases flow-rate. However, it has absolutely NO impact on the head height! Let's also remember we are dealing with both static friction and intertial friction...

IE:

Fill two buckets of water with a hose in the bottom connecting each to the other. Now place your canister filter and its media in-line with that connecting hose and fill one bucket. The water will quickly find its own lever regardless of the medias resistance to flow. The media reistance may slow the FLOW-RATE to a trickle, but it will not affect the HEAD HEIGHT of the water. A pump merely increases the head-height on one side of the hose, causing pressure, which then increases the resistance to flow due to plumbing restrictions and/or media, as you pointed out, but it will still reach that height (more or less depending on pumbing size and other factors).

I think we understand what each other is trying to say.... just missing the connection here.

DogWalker
12-16-2006, 11:32 AM
Originally posted by Glaive
So the reason for a lower flow over a longer system is not head height but the friction of the system. It makes sense.

Totally agree that longer hoses would have this effect. We were overlooking that part for argument's sake here though, and focussing on the height impact. It would be possible, but not practical, to do the 100' test fairly, by using the same length hose no matter the height and taking great care not to have more curves and bends at either height.

Look folks, to spare you all some grief here I'll run a test next time I do maintenance on a cannister, to satisfy my own curiosity. I don't want to push this to the point of starting a quarrel. I promise to do the test as fairly and objectively as possible and report back no matter the result. I'm never ashamed of admitting it if I'm wrong - but it can sometimes take a lot of convincing . :wink:

RustyNut
12-16-2006, 03:32 PM
Hey, I'm not quarrelling, I hope other readers are learning from our good natured debate. As long as we are learning I am enjoying it. I can admit to being wrong too, just in this instance, I know I am not. :)

DogWalker
12-16-2006, 03:45 PM
I always try to keep an open mind to the possibility that things may not be as they seem to me, and appreciate that open-mindedness in others. I always viewed this particular application the same way you folks do until I started thinking about it more - a simple enough test will tell me if I took a wrong turn there.

Anyways, I enjoy a good discussion of this nature as well. Now when somebody wants to start opening up my arteries to illustrate their point I'm not so sure they're enjoying the debate :eek:

gaines
12-16-2006, 04:58 PM
Originally posted by DogWalker
Now when somebody wants to start opening up my arteries to illustrate their point I'm not so sure they're enjoying the debate :eek:
Hey now...it was just an illustration...although if you're volunteering... :sygypsy:

DogWalker
12-16-2006, 05:13 PM
LOL

cich1
12-17-2006, 04:00 PM
so let me ask this question. maybe i am just thick headed but what you guys are saying is that head height won't affect the pressure on the output? i don't know if i wasn't clear in my original post but that is what i was getting at. i thought that the farther the water had to travel up it would decrease the output pressure. forgive me if this has already been explained , maybe i am just not getting it.

Glaive
12-17-2006, 04:30 PM
The physics behind it is roughly the following:
There are equal gravitational forces pushing against both ends so they equal out. This being the case the only force acting on the system is pump.

RustyNut
12-17-2006, 04:56 PM
Just ot clarify more...

The pump can then pump its head hieght above the waters surface (minus plumbing losses) - but this ONLY pertains to a closed system like a canister.... in a sump, the pont used is the sump surface......

cich1
12-17-2006, 05:04 PM
okay i get it now, thanks guys.:ok:

DogWalker
12-30-2006, 01:35 PM
Ok folks, just to keep a promise, the Mythbusters have finished their work on this. And the results: output from a Rena XP3 at floor level and at 26" above floor level was the same, using the same hose lengths and roughly similar curvature. Had there been a difference I would've repeated the test without media, just to see, but as it turns out that was unnecessary.

Gaines & Rustynut, no disrespect, it's just that seeing for one's self is the surest proof. (I work in an environment where some pretty bright people occasionally turn out to be wrong on 'simple' things they were 'sure' of).

Dug up my old Fluval manual to see what the heck they were talking about - it turns out the table in question is purely an illustration of loss of output as head height increases for the pump itself - it has nothing to do with the application in which the pump in their product will be used. To include that in their manual is misleading (perhaps the Fluval subject matter experts dropped the ball on that one :wink: ).

Anyhow, you guys did indeed have this one covered - case closed.

MisanthropeKitty
12-30-2006, 01:57 PM
my head hurts.

you guys do a wonderful job at confusing the hell out of me and making me reread everyone's post at least one time. But when it comes down to it y'all always make perfecty sense. Great job guys!