We have one 55-gallon set up, and just sold another 75-gallon one because there just wasn't anywhere to put it in our house.
At one point it held a dozen or so different rainbow fish plus a single German blue ram named Heimlich, but the last rainbow died a few weeks ago, and now it's just four neon tetras and two plecos. Lots of space for the neons!

We'll get more rainbows soon, I think. The ones who died recently lived almost four years, despite the fact that we never do water changes, ever.