I usually aim for right around 78 or so. Higher temperatures will effect the amount of oxygen your water can hold, so the higher the temp in the tank, the more surface agitation you should have to keep the water oxygenated. Higher temps also mean accelerated metabolisms increasing the amounts of contaminants your fish will release into the water prompting a possible increase in frequency of water changes. Temp also tends to help some fish with spawning, but I think that's more species dependent than anything.
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