Fish Dying But Not Sure Why?

[nykki76]

Hi,
Can any suggest what may be causing my problem please?

My tank if about 5 months old, its 170 litre tank. I have bog wood and plants in it.
I have around 40 fish, some of which have started gasping for air at the top, but not all. My white fin tetras, mollys and sword tails seem to do this. The other fish not, i have pentoza barbs, pepper corys, albino corys, 2 bristlenose plecs, a singapore shrimp and couple aquatic frogs, neon tetras and a pair of blue rams
I test the water every day, no ammoina, no nitrite and a low level of nitrate. I've found myself water changing most days 25%, then they all seem fine till the next day.
I bought a couple of new fish at the weekend, 6 harlequin rasers and 6 white fin tetras. Yesterday one of my pepper corys was floating upside down at the top, but not dead untill this morning. Then i get home from work today and all my harlequin rasers are dead along with one tetra. I dont know whats happening or what im doing wrong? what can i try next?

Many thanks
Nikki

 

[N0body Of The Goat]

What temp is the water being kept at?
What powerheads and filters (rough litre per hour rating would help too) are rippling the water surface?

Your issues may well be disease related, but there is also the chance this is down to oxygen levels in the water, especially when you say the fish seem fine just after the water changes. Water can hold more oxygen at lower temps, providing the water is rippling enough to help saturation, which will help some of your fish like the Peppered Cories would appreciate (they prefer low temps around 20C over typical tropical temps of ~25C).