Empty tanks, need ideas

You could try African Clawed Frogs. They are completely aquatic and don't need any land area. I kept a large one in my monster tank with a small peacock bass, rtc, spotted gar, and even silver arowana but I gave him to a friend because my fish started to grow faster than he would. They will eat basically anything that can fit into their mouths, bloodworms, live fish and crustaceans, earthworms, and mine ate pellets sometimes. They come in brown and albino and you can keep several of them together.

You could do a poison dart frog enclosure with a waterfall but don't use a screen lid because they need humidity