Here's another crazy idea. How about a series of pools overflowing into each other, built out of grain bin rings? They're pretty cheap, especially if you can find them used and you could have pools for different species. Liners can be purchased for them. This is how my tilipia tank was built and I've seen similar at fish farms. You could then backfill the sides and finish the edges to look natural. They typically come in 33" or 44" high if I remember correctly. They wouldn't need a foundation either.
