After you told me that comets get really big, I decided to get a new tank for my black moor the next morning, they both live in 21 litre tanks now. In a couple of months I'm going to get a bigger tank for my comet. At the moment I'm maintaining both tanks with the same levels of Nitrite and ammonia which are 0. Ph level is 7.5 for both tanks and the temperature is 22 Celsius for my comet and 23 Celsius for my black moor. My black moor isn't swimming the way he did yesterday.