I use those big black nylon rubbermaid troughs - anywhere from 55 to 100 gallons. I keep them in the shade and always keep a pair of koi in them and the trough is on an automatic waterer. Water stays pretty clear (koi are in the carp family - they love vegetation). Take the koi out, algae blooms - put the koi in, crystal clear - I've tasted it myself - not gross at all. So I vote for fish! Koi can get 2-3 foot long over time, so they must be able to be moved to a tank or pond that has grass or vegetation growing in it...but they are WAY more efficient than goldfish. A pair (under 6" each) works well for the size tanks mentioned above - they never can quite eat it all so there is always something for them to eat.
Koi are worth alot of money when they are over a foot long, too. Some ornamental gardeners will pay upwards of $300 per fish (depending on the color) so you can always put an ad in the paper when they get too big... who knew, right? LOL! Then start over again with some more koi...haha!