The cause of a loss of appetite in tropical
fish is most commonly stress. New fish which have just been
introduced to a new aquarium
are prime candidates to exhibit a loss of appetite. Your tropical
fish can also feel a great deal of stress due to overcrowding.
If you have just introduced new fish into the aquarium and
the original ones are exhibiting signs of stress this may
be the case.
The only real thing that you can do to alleviate this condition
is ensure that there is no parasites or bacteria present in
the tank. Also, ensure that the overall water quality is good
in the aquarium.
If you take a sample of your aquarium water to your local
pet store they will likely be able to test it for you to determine
if parasites or bacteria is present. From there they should
be able to recommend a solution to you.