Question
i have a 30 gallon with 2 comets which are about 2 years old. i test the water weekly- biweekly and change out water according to the results. their water tests well right now, but one of my fish is sitting on the bottom more than usual and the other fish hovers just above. the only other change is some brown algae on the artificial plants. what is going on in there?

Answer
What do you test for?  I'm sure you do not test for things like oxygen, total organics, pheromones, nitrate, etc.?  Probably just pH and ammmonia?  Just because the water is ok as far as a few tests go doesn't mean that it's not bad in other areas.  I do weekly 50% water changes on all my tanks.  I suggest doing routine water changes even if the water tests just fine.  I only test my water if I suspect a problem.  When fish lay around, it often indicates either low oxygen levels or high organic levels from all the waste.  What kind of aeration and filtration do you have?  Add an air stone if you don't have one.  Fresh activated carbon will help remove excess organics.  When you do your water changes, don't forget dechlorinator and aquarium salt (a tablespoon per 5 gallons) which helps with many problems.  Good luck!