this is way late but FWIW, minimize the rice grain to the smallest number of polys – use textures to make it appear smooth and with whatever detail you need. Second, cheat. Do not attempt to fill a large tank with something the size of a grain of rice just make it look like you did (unless you have to actually show that – but it’s not likely you really have to). It doesn’t have to be a full tank of rice (or whatever) to stick a hose into to suck it up only enough actual particles to show the pick up, what’s under that can be a filler solid object that is never seen or has a texture on it to look correct if it is seen…