Yes, there needs to be lots of movement and water circulation around the sand, so much so that it 'tumbles and bubbles' like you said. Key is keeping that movement and not allowing any areas to compact. Well oxygenated water should circulate continuously around the sand and feed the nitrifying bacteria colonizing the sand surfaces.fishwolfe said:is the sand in a fbf supposed to tumble or bubble around?i have a d.i.y idea and just wondering about how much flow needs to go through the sand.