Lost another Poppondetta

[Lithril]

Lost another Poppondetta over the weekend, went away from Friday till yesterday and found another Poppondetta had died. The nitrite levels are again between 0 and 0.1 so I was going to change water anyway. Took a water sample into the lfs and found the ammonia levels were fairly high. Will continue partial water changes for next few days, I've also bought a new filter (Fluvel 2 plus) as I havent got a lot of faith in the old filter (interpet combined heater and filter). The build quality on the old one is cr** and theres got to be a reason why the ammonia levels are still creeping up after 2 months.

If anyone else has any other ideas it'll be much appreciated as I wont be getting anymore fishies until I've got the tank stabilised.