I've always liked koi ponds but have never had the chance to have one. I recently moved and there is an old above ground pool out back. The liner is ripped up but the frame is there. It is 17 feet across and almost 4 foot tall. I think that makes 5967.85 gallons of water. If I did that wrong, please let me know.

1. How many koi could be kept comfortably in that?

2. Can catfish be kept with koi? My dad is wanting to use it like a stock tank, I want koi.

3. It's behind the shop and we were going to collect rainwater to use. I assume I would still need a filter but would I need to treat the water?

Thanks