Can I put fish in a tub or sink?
Yes and no! The larger the container the better for the fish. Bear in mind above ground, pools get very warm and very cold quickly, and gives a large temperature fluctuation. est fish are goldfish and shubunkins. Fantails are lovely but should be removed for the winter.
In practise, I would remove all fish for the winter. The containers will freeze around the sides as well as the surface. In a tub or a sink with a larger surface area, a very small fountain or spurting ornament can be installed. Remember lilies don't like too much moving water.
You may also be able to fit in small growing marginals. Plant up using the small plastic containers available. If they are too deep, cut them down a little.
Tubs & Sinks Advice
I doubt if there is a place in any garden, patio, decking or balcony where a small tub or sink would not make an attractive feature. Television gardening programs are awash with the idea.
It does not have to be a tub or a sink. Look around and imagine any container that holds water to be a possible feature. All that matters really is it be ideally no less than 20cm deep and frost proof! Most people think of wooden barrels at first. These come in various sizes and depths. Ideal for a pygmea or very small lilies, some oxygenators, floating plants and possibly a deep marginal.
Site the feature for full impact. Even brighten up a dull corner! The good point about this sort of pool is if you don't like it where it is - move it.