Mosquito fish (and there is no shortage of information about them
on the Web) are similar in size, shape, and fecundity to
guppies...they look like greenish guppies. They don't foul the water
like goldfish, and you don't need to feed them. They live on insects
that land on the water and whatever algae and stuff grows there as
well.
As they're pretty hardy and self-replacing, I've got a bunch to
spare. Email me and I'll give you a phone number and address, near
I-17 and Northern.
A reminder -- DRAIN your evaporative coolers when you're not
using them (like right now, I hope) to prevent raising your own
batches of mosquito larvae or Legionellae.