I have a 29 gallon tank with 5 fancy guppies (2 just had fry) 3 neon tetras, 3 glolite tetras? i think thats what there called a bala shark and i had 4 platys. last night 1 of them died and this morning 2 more died. there is one female left and she seems to be doing well. I have no idea why these fish died. the tank has been running for around 6 months now and ive never had a problem. all the other fish seem to be fine. is there something i should look for.