FW jellies are tiny. Thank you all for your ideas, but we have a winner. It would be criminal to not keep jellies in this thing, and I live 14 miles from a lake known to harbor them. I'd be an abject moron to avoid this journey. I'm calling my one friend to see if wants to go critter-buggin' in the lake with me. Even if we don't find the free-swimming form, I'll probably get a few in the muck that will transform for me later.
This greatly simplifies everything. Now the DSB with an airstone makes a whole lot more sense. Jelly turds ought to be planktonic enough to sift into the sand and be digested by anaerobes. And with such height, the bright light I will use to grow green water won't overheat the little guys. And now I can totally do some kind of plant. Hopefully the native water weeds there will like life in the tube.