Q. I have  questions concerning the behavior of Oscars. I just want to make sure they aren't sick. First of all, everynight, when it would appear they are sleeping, they actually lay down on the gravel. I have  never seen a fish do this. Everytime is see it I think they are dying, but everyday before I go to work and when I come back they seem very active. They only seem to do this at night.. or when there aren't any sources of light. Another thing is, they seem to change color. I have 2 normal (grey) oscars that can turn as white as snow one minute and jet black the next. I have asked some people about this and they look at me like I'm making it up, but I'm not. My temp is 80 degrees, ph is about 7.0, there isnt hardly any ammonia in my tank. I do regular partials, so I'm sure the nitrates are low. Does anyone know if this is something I should be concerned with, or is it normal. I have had them for about 4 months. the aquarium has been setup for about 6, I use a under gravel filter system with a Penguin power head attached. Thx for any help on the matter!

A.  Without lights an Oscar as well as other fish will 'sleep' or become inactive. They will sink to the bottom then lay on a side. Changing colors during this period is perfectly natural. They will  undergo color changes during the day as well. This has a lot to do with mood. It can, however, be related to stress.

Getting a nitrAte test kit to test the water is always a very good idea. Oscars are very messy fish. They produce quite a bit of waste, and water quality can suffer as a result.

Since I am just guessing, I will say your fish are fine, but a nitrAte test would help shed more light on the situation.

Also, you may want to consider changing out that UGF. An Oscar digging in the substrate will pretty much render that thing useless. A decent canister filter or HOB like an Emperor setup would be a much better choice.

 - solace