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I'm not OP, but look for stock tanks. For filters... so many options, but most would use a pond filter outdoors. Kid pools are OK, but I haven't had luck with them. Algae... a well stocked tank should not have too many problems but you can use a UV sterilizer if there is a severe problem.

That softshell turtle though; it's certainly a rarity to see here, least in the part of the city where I live.

The pond looks really neat! You're turtles all look happy and laxed to live there, together with the fish. Turtles do have fish in their diet, but I'm guessing in your case the turtles are being feed enough to not chomp on the small swimmers, or it's the milk crate that also acts as a haven for them.