Fish keeper, have you seen the videos I posted of the paddlefish? Their first tank was a 30 gallon tank. They are in a 75 now and have all but quit growing. I have an inch of black sand in the 75 with some sparse driftwood across the bottom and a large chunk of coal i found on the beach, a triangular piece of conglomerate rock (green with white inclusions) from the French broad river and a large chunk of white polyester polymer. I filter the 75 gallon tank with an Emperor 400 one one end of the tank that causes a continuous current across the length of the tank. That set up gives them two currents, one on the surface and a current near the middle coming back to th filter.
Paddlefish will ride these currents as they continuously swim, some times they will dip down and swim among the rocks near the surface of the sand, I haven't seen the paddlefish interact with the fine black sand substrate yet, but they do swim among well placed decoration, like the rocks and wood in my 75 gallon tank. They do how ever spend most of their time in the middle to surface layers, the good strong current of the surface lets them swim slowly and ride it back down the tank.
The large area of the Emperor 400 filter lets the water flow over a large area of the water and this makes a good current for the fish to swim in. I put a sponge filter over the intakes to keep any daphnia or small pellets to be picked up by the filter.