Fish will likely lead to increased levels of phosphorous, waste, and parasites, and may not be a great option for your regeneration zone. If you're deadset on having a few fish to swim nearby, choose small, low-impact options like goldfish.
While fish may not be an ideal option, your regeneration zone will attract its own kind of critters. Tadpoles, dragonflies, snails, frogs, and other small creatures will do well in the conditions your mini-ecosystem provides.
