Put them in your 90 gallon.
It's way too shallow for overwintering fish anyway. IMO herons are worse than cats.
I hate herons. They always kill my bass. They piss me off. I probably lose over 50 bass a year to herons. And thats in a big pond. Could you imagine in a small pond like this one? Its like the bass are practically in a bucket. Id make it as deep as you possibly can brandon. To avoid birds and cats and still allow for the fish to thrive.
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I am not sure if this will work. For the winter build a insulated plywood box, maybe a clear top for light to get through and run a duct to your house. You can just use the house heat to heat the air around the pond. You can even add a heater. 3 feet isn't far. It won't be tropical temperatures but good enough to keep them hibernation mode without dying. Just monitor the temperture and if it drops to low bring them in.
50 bass?!?! I'd be so mad if i were you. My neighbor used to have that problem until he put some decoration there. With your pond being like 10000000+ gallons, and mine being 200(used to be 280 without the many bags of sand displacing the water). I wish i could make it deeper but the liner is in, the sand is in, the rocks are around the edges and the plants are being added. There is a 2' x 3' area thats like 15-20" deep. Also it is 3' from a big window in my house. Maybe thatll scare them.
DUDe those birds are probably helping you more than anything.
When a pond or lake gets too many fish in it they dont grow as fast. TOo much competition for food. Unless the pond is given an additional food source those herons are just helping you weed out smaller fish and letting the others grow faster just like they would in mother nature. Just consider them as some cool decorations. You will appreciate them more as you get older im sure. Birds gotta eat too.
FIsh if I was you I would drain that bad boy and re dig it. MIght as well while you still have time.