250 gallon converted 'tank'

[Bombalurina]

If you like the idea of natural filtration, you could try and turn that 250 into a reed bed. I hear they are pretty good - good enough that people use them to purify waste from their houses.

[Backlash]

You could use the 250 to breed Koi to help fill the 500..?

[Sena Hansler]

Why not try out minnows? generally hardy, and are not tropical. They come in different colors, and some often then not do get bigger than the usual minnow ssome people are used to.