If you add some fast growing plants, and only stock small fish you will be fine without filtration. I have a male and couple female halfmoon bettas in a "pond" about that size that I'm trying to get to breed. Of course around here they can only be outside a couple months, but they were breeding til it rained and ruin the bubblenest. (silly me for not covering the nest!) Considering where you live just about any tropical fish should be fine in the summer. Rosy red minnows, barbs, tetras, livebearers, rasboras, loaches, corys, etc. Just make sure you know the species safe temp range and bring them in before the average temps go outside it.