You might wonder how my three-year-old is able to part with these creatures after we watch them for a while ... Well, at this age it is fairly simple ... you just tell them that the frog/fish/mermaid really misses their Mommy and Daddy and so we have to put them back where their parents can find them. Isaac totally gets that, and would never want to keep a creature from its Mommy.
Anyway, friends of ours have started their own "unschooling observatory river tanks" as well. Their tanks were also yard sale finds ... and are currently housing several river fish.
This is such an inexpensive and fun learning experience. No filters or other tank gadgets. Just a tank, some river/pond water and fauna & a bubbling do-dad thingy (for circulating the water). You can feed your fish with commercial fish flakes (as my friends do) or you can just keep replenishing the water and animals so that feeding isn't necessary (what we do).
Let us know if you give this a try.
Enjoy your day.