How about using them for some form of fermentation? Wine, Olives, Pickles? I'm not sure what they look like. If they are what I imagine, I instantly thought of Viking and Roman Archeological digs for Wine, Beer, or Mead.
I like your thoughts. I would combine some of them, though.
What would you think about growing an aquatic system out of your tubs, with a view to using the fish water to water your soil-based gardens? You could use some flow-form ideas to mix things up and oxygenate properly, or just have water cascading from the low corner (you'd have to situate the tubs so that they cant ever so slightly to one corner) of the higher tubs to the lower ones, with the flow landing on and percolating through mounds of rounded stones or something.
With three tubs comprised of five individual basins, you could experiment with greywater filtration and treatment in one or two small basins, complete with a functional reed bed for filtration and treatment, before the treated water proceeds onto your aquacultural setup.
You could have a nursery pond that scales up until the last two full-sized basins, where adult fish are grown out. It would also be possible to grow certain veggies hydroponically, floating on rafts with holes cut in them, such that the root zones are submerged. I know that some head lettuces can be grown this way, butter crunch, boston, and romaine among them.
Ponds and water features also benefit many pollinators and predatory insects, as well as some amphibians and reptiles, and the presence of so much water and mud will be a boon for anything making its nest out of mud, as some solitary bees and wasps do.
Lastly, if you didn't have the room to display them but wanted the water storage capacity, you could bury the tubs in a walkway or something, link them, and construct lids for them, like a mini-boardwalk. You could ostensibly still keep certain fish and plant species if you were able to create light tunnels that would illuminate the basins under the walkway, but simply using the buried tubs as a cistern, even if they filled from the mains, would enable you to let that water come to ground temperature, at least, before being used to water the plants. Contrast this with the temperature of water coming out of the mains. Also, having a cistern like that would allow some of the chemical additives in the water to offgas.
Let us know what you do, and good luck!
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