I use a @1000 gal kiddy pool to over winter water lilies, goldfish and koi in my basement, and put it outside in the spring thru fall. At slightly over $100 very economical, and with its ease of breakdown and setup, I find it very useful.
The large surface area also has great potential for gas exchange.
Here it is in the basement

and outside in the summer, this year with Coptodon (Tilapia) zillii

I also put a 500 gal in my sons bedroom when he went off to college, for cichlids too big for my tanks