1. There are many fish.
2. Lower the water inlet near the bottom of the cylinder.
You can add pipes.
3. Changing water is a bit frequent, which is not good for fish after a long time.
4. Has the filtered cotton residue been washed?
Fish will not sink to the bottom. It is normal. You feed koi fish, a benthic fish with a wild temper. Come up, train them, let them look at people more, and don't scare them.
Feed it several times at a time. People always wait until the fish come to eat before leaving, and on this basis, increase the influence on the fish, so that the fish are not afraid of people.