U will need to find the depth, that will tell you if it's deep
enough to sustain fish during the winter months. Also do you have algae growing in it, this will help tell you about oxygen that's in the water. I go to our county extension agent and ask for advice and help, here it is free for the advice and they will some times come out and see what you have. Depth is a big concern though, even in the heat as well as the winter. I don't know if you have ever used a sane, but you might get a cheap one and throw it out in the water and see what you retrieve. Blue gill or red ears, bass as long as you keep them under control. And cat fish, they can survive in about any kind of a pond. Cat fish are tough, if you just want to raise something to harvest. We had an ole pond that wasn't very deep and it was small, we threw cat fish in it and fed them just like we do the live stock. Fun to watch them and also see how big they get. I didn't read down far enough to see you had already was going to check with your extension agent, I do know we put some chemicals in to control algae and I just lost what the other name of the stuff was that we had growing in our pond.
Must be my age, that's my excuse any ways.