I just found a supplier for 18"-30" (or up to 50 lbs) paddle fish. I don't know how much interest there would be in them though. Still waiting on final pricing, but should be much less than the ~$3 per inch that has been described here. 18" minimum would avoid predation fears also unless you had some 20lb Blue Cats in there.

Would many people be interested?

They feed exlusively on zooplankton and gather pytoplankton incidentally. They should be a good way to reduce some over abundant fertility without causing any harm to the base of the food chain.

Transportation will be fun to figure out since their gills do not articulate. I would need to devise a way to create a moderate flow in the tanks.