Cold Water Bottom Feeders

Over the past few months I have been helping a friend with some indoor ponds at a building he works at, these ponds were seriously neglected by the previous building operators. I have been working very hard to get both the ponds and fish in better condition. I have taken the majority of the fish home to care for, since most of them are koi they will need to be rehoused in the spring, I'm planning to keep them with me until then. The rest of them are sarasa comets, and they are going to be put back in some of the ponds in a few weeks. Right now I am working on the largest pond, which is 1,500 gallons, we have decided to go with a variety of fancy goldfish, I am growing them out now and they should be ready to go in their new home in a few months. I'd like some bottom feeders of some sort, the only ones I can think of, that are suitable for the pond are weather loaches, however, my friend finds them to be rather ugly and doesn't want them in there (frankly I think they're quite cute, but it's not really up to me). The pond is not incredibly deep either, it varies, but I think the deepest part is about 18". The temperature of the pond is usually at a stable 66F. Now, on to my question (sorry for my rambling), does anyone know of a cold water bottom feeder that would be suitable for this pond other than weather loaches or common plecos? Preferably something that won't exceed 12" and will be happy in there for life, also, hopefully nothing too expensive since I'm doing this out of my own time and money, because for some stupid reason they don't have a budget for the ponds other than the cost to run the pumps and filters. I'd prefer no snails either, and the reason for the bottom feeders is not because they're overfed and need something to clean up the mess, mainly just because I really love catfish and loaches and I thought a large group would look very nice (just thought I'd mention that in case someone may come to that conclusion).

Thanks for reading and hopefully replying!

Perfect, thank you very much. I am hoping to get a heater in there, but like I said, there is no budget right now and the electricity bills are closely monitored (or at least, so I'm told, I think he's just playing it safe). The point of doing this is to get these ponds looking healthy and pretty so they might be more willing to consider a budget. Anyway, I will do some reading on North American catfish and see where that takes me. Thanks!