Alot of people dont realize that benifical bacteria lives in the gravel and uses leftover food and poop to break down into harmless nitrate. You can use those kind of round river stones just fine, just watch your feeding and add Stress Zyme with your weekly water changes. Ive tried them all, pea gravel, Sand, Round River rocks. You can even use dirt with a layer of sand or gravel on top, its just a matter of personal preference of how it looks and maintianing good water change schedule and adding Stress Zyme to get the Benificial Bacteria going good. Its how it works in nature and dont overcrowd the tank or over feed it. Some of my tanks have Just an Undergravel filter and my 40 Breeder tank has just the rocks, plants, and a airstone. Just tested and there is no ammonia in it. Just got to feed lightly 2-3x a day. And only exactly as much as they can eat in 2 minutes, Fishes stomachs are small but they are ready for more food in 3 or 4 hours.