Dear Gold Fish...

For this 2 weeks,4 of my fishes died in the tank.One died today,found out with I got home-floating near the air pump.What's wrong?The water is not poisonous isn't it.They are being eaten by the live fishes when they are dead.Even more poor thing.Oh my god!What happened?Now currently there are 4 more goldfish left.I think I am going to add more fishes into it.
Maybe clown fish,fa cai yu,golden dragon and angel fish.Do you want to buy some for me?