sick bala shark HELP

[davidefc]

ok i have had 2 bala sharks in my tank and both wer puchased at same time and size but 1 hasnt grow hes the sick one and the other 1 whos fine is about 2-4cm bigger and thricker, they get on fine but the 1 who hasnt grow is looking realy weak and swims upwards and downwards and go's around the tank. what is wrong with him all the waters fine expet the nitrate which is about 30-50 which is falling quite fast as it was 100 at 1 time.
please help david

 

[Steve737]

Do small water changes often to lower the nitrates. Don't do 1 big one, it will shock the fish; but a series of smaller ones.

 

[davidefc]

.. i did the compleate opposite i did about 50% 2weeks ago 60% last week and about 30 today before i posted this question. do you no whats wrong with it and why no groth at all.

 

[davidefc]

any1 help, hes still alive just.