I'm a newbie into the fish world after winning a Goldfish at a fair back in early November. The day after we went out and bought a tank (30 litres) and on the instruction of the guy in the pet shop put our new pet in straight away as living in a bowl was far worse. And our fish was perfectly fine. We waitied about a month and then got another fish to keep him company and all has been well up until about 2 weeks ago. I have regularly changed the water at about 20% every week since (using API goldfish aquarium cleaner and Goldfish protect in the new water and a small dose of aquarium salt) and been testing the water each week as well. The tank has a rena superclean 40 filter and we've been using API Crystal dose which I have changed every 3 weeks as instructed. Recently I have noticed that the Nitrite level has gone shooting up and that the newer fish has been floating to the surface, gulping for air. Having done a bit of reading I opted to do more frequent and larger water changes in order to get the level down again. This has not been successful, as it is still very high and now our original fish appears to have fin rot as has a whiteish tint to his tail fin and is also very larthargic (this has happened in the last week). I am hesitate to just add chemicals for the sake of it and would like some advice on what to do from now on.