I'd second the medaka rice fish suggestion. I find mine adorable, in a planter. They live in paddy rice ditches and are kept in quite small pots/planters in Japan. I think they'd adore that set up. They are hardy and make friends with their keepers more than almost any other fish I know. If you are ever in Edinburgh I have some of the platinum that you can have. They are quite small. They have bred once for me. Another species is Least Killifish. They are quite slow, and not as attractive, but also hardy. With the medaka potentially you could have a planter for them to use in Summer, if you can accommodate that – in a garden, on a landing, on a verandah?