Sick Fish - Can't solve the problem

[Myka]

I'm confused, is the 300-gallon the display? It's the display that has 30 lbs of rock? If you don't have enough rock you will need ceramic rings or bioballs or something like that. Foam is a bad idea because it gets clogged and can end up being anaerobic (bad). Do you intend the tank to be fish only or a full reef? If you want a full reef you will need rock rather than bioballs/ceramic rings, but if it's fish only then bioballs are very good at their job and inexpensive.

How big is the treatment tank? CP can definitely affect biological bacteria if you use a significant dose of CP. If you stick to usual dosing of 10-20 mg/L it shouldn't affect biological bacteria. What do you have filtering the treatment tank?

Also note, ammonia detoxifiers are NOT compatible with Salifert test kits (I think Elos is the same, but I haven't checked specifically). You will get a false positive no matter how much detoxifier you use. The SeaChem Ammonia badges work well (I've used literally dozens of them over many years) and although they work very well as "alerts" they are terrible at giving an actual measurement, so don't rely on it for the amount of detoxifier to dose. API test kits are compatible with detoxifiers, but API isn't the most reliable test kit...