He's building a small lake in the backyard! Not sure how I'd feel about my husband doing something like that. He's lucky to have such an accepting girlfriend
I would also guess it's an oxygenation issue or build up of nitrates in the water from serious overcrowding (I know it is a temporary situation, but it is likely very overcrowded nonetheless). How's the filtration in the holding tank? I would imagine with a population that large, he would need to change the media far more frequently than in a decorative fish tank. Also, has he added an adequate air pump to the holding tank? I think some species of catfish can breathe air, but not all can (and I am not sure what species those are). That's where I would start- filtration and oxygenation. I'd also be sure the water temperature is adequate for the species and add heaters or chillers if necessary.
How long until the pond is complete? Since the fish like them, will he be adding plants to the pond?