Hello, I recently got some cardinal tetra. The first couple of days everything was fine, but now they're breathing from the surface. I lowered the temperature and put on some extra oxygen. The nitrite is low and it's about 24celcius in the tank. Any idea what the problem might be? They don't do it all the time, but they've done it two mornings in a row now. The oxygen addition is off at night, so I can sleep. Got something to do with that? The other fishes seem unaffected.