Porlly a sign of ammonia build up. Do a water change asap! Be sure to add dechlorinator
the pump pulling to strongly will not hurt a healthy fish. There was something wrong with it. It is probably b/c you started with two many fish in an uncycled aquarium. There is alot of forums that discuss cycling a tank, but I'll give you alittle info. on it:
What you do is get your tank and put EVERYTHING you want in it EXCEPT FISH...
buy an ammonia, nitRITe and nitRATe test. (there wont be many test kits that testy all three, you'll prolly have to get atleast 2 diff)
Buy some ammonia that says CLEAR ammonia, no other kind of ammonia will work.
Pour some in the aquarium and do an ammonia test, until ammonia reading is 5.0 or just slightly less.
then test once a week until ammonia starts to go down, then start tests you Nitrites and nitrates.
when ammonia reaches 0.0ppm, you'll need to start pouring a little bit of ammonia in the aquarium to keep your bacteria alive..
When both ammonia and nitrites are at 0.0, test your nitrates. They should be WELL into the 100's-200's
do a 75-80% water change and make sure that nitrates are below 20ppm.
then you can have fish
[this process usually takes 30 days]
or you could get a fish that is hardy and monitor water stats being sure that you keep ammonia below 1.5 and nitrites below 0.5ppm.
Hope this helps