Fish keep dying in tank except 1 common pleco

[rogerseah79]

Dear bro and sis,

My 522 tank fishes of parrots died off in CNY and left 1 common pleco still swimming in it till now.

I have changed the water and cycled it till now with the pleco in it.Today I bought 9 common kois and introduced them in and they have since died off till left the 1 common pleco still surviving.

Please advise what is wrong with my tank?I am running with a canister of 1400l/hr.

Thanks in advance.

Roger