Thanks for your suggestion but the substrate will be vaccumed because it will get dirty if I get lots of baby fish, so the bottom will still get stirred up.
I have read that there are two types; abrasive and smooth. I assume anything sold at a pet shop is the smooth stuff, and that most hardware and gardern shops sell the abrasive. Does Placemakers sell both or just the abrasive one? I have to be very careful with what I put in because the Apistos will sift through it and I don't want them to get hurt.
Apart from silica, what other types of sand are available for aquariums that don't leach anything into the water or alter the pH and hardness? Keep in mind that it has to be non-abrasive for the Apistos, and no darker than the stuff I currently have because in an Amazon biotope the sand has pale colouration.