If it were me I would clean the muck out of the pool and add a liner, then extend the water inflow down close to where the pool overflows and exits.
You mean the whole pool, not the tank, right?
Yes, the old pool/trout pond, not the galvanized steel tank.
Now if memory serves me right fish can have issues with galvanized steel but I don't remember the particulars.
I have thought about framing the inside and having concret delivered to repair the pool but it's a pretty big job for a weekend place. It would probably be costly too. We did use some kind of plastic sheet years ago and fill it up. The plastic eventually started leaking so I removed it. One day, when I have some money I'd like to have it repaired properly. I can't afford it right now.
