I've heard someone say we are not fish keepers; we are water keepers. What works in one scenario with one type of water will not work for the next, or for that species of fish or even for that particular fish's immune system. And some of those books are written by companies that want to sell...
Sorry, very long response here. TDLR at the bottom...
Changing the filters is actually one of the worst things you can do in a tank. That brown scum that builds up on your filter media? That's the beneficial bacteria that deal with your nitrogen cycle. If you had nothing but sponge in your...
It could be the angle of the picture but it kind of looks like he had a crooked back. If they have a spinal deformity they can be very prone to organ failure... a lot of the time it's caused by inbreeding from stock at major pet stores.