MY TETRAS Keep dying.

Question
There were 300 fishes in the tank.
The tank is 4ft big.
3 types of fishes, Neon Tetras, Red Noeses, Stamp fishes.

Most of the fishes are together, they swim in big group. Which is natural i
know from experience. But then there are fishes which swim isolated(quite a
large number). The fishes fist turn slightly white, then they do sudden
movements like turning upside down then almost immediately they start
swimming properly again. Then gradually they stop swimming properly turn
completely white and die. Fishes from all three species die the same way.
My filter is rather good. There are sponges and all the required stuff to keep
the water clean. A fish shop person suggested that the fish may have dies due
to itch so i put in some itch medicine (fish one) into the tank but it didnt help.
My ph meter shows a reading of 5.8 to 6.5. I really dont know what to do now.
I feed my fish proper tetra food. I have kept fish for the past years and
nothing like this has happen before.  

Answer
Abilesh,

 You are grossly over populated. Why do you have a sponge filter in the tank? Do you have fry(baby fish)? What are the ammonia, nitrite, and nitrate readings. I am thinking they are suffering from either ammonia poisoning or nitrite poisoning. Do you have an air stone or a few in the tank? Unless you see an illness never treat a fish with any medication. If you do, remove the carbon from the filter. If you leave it in the tank it will remove all the medication you are trying to heal your fish with. I would remove the sponge filter, since I don't think you have any fry in the tank and use filter that hangs in the back. With that many fish you might want a few filters in the tank. After you have tested the water, let me know what the readings are. As soon as you get back to me, the sooner I can help you out.