Prime is a conditioner, but it also binds toxic chemicals for up to 48 hours, which would be hugely helpful in a situation like yours, where you're so overstocked. If there's a possibility of obtaining it, it would be great, but if not, you're still taking excellent care of the fish. The problem is that four such huge fish in such a small tank have to be creating unbelievable levels of waste. I know JLK mentioned not moving them to the pond yet because of stress. Is there a chance you can get a large Rubbermaid tote to put two of the fish in? They're quite inexpensive and make good temporary fish homes with a filter and an airstone in them and that would take 50% of the stress of the bioload in your 29 gallon.