One of my zebra danios is laying on the bottom of the tank. I had a hard time finding him in the tank so I added some food. I noticed he was swimming erratically and then would slowly sink back down to the bottom. He's specifically laying on his right hand side. I separated him from the tank and put him with half old tank water and half new water and added some prime in case it's an ammonia spike. We've got four total zebra danios, three leopard danios, and one tiny guppy. He's still laying on his side in the smaller tank and will occasionally swim around before sinking back down. Any ideas? Is he doomed?