The wife loves the look of sand so we've also done it in the 4ft tank. I kept most of the gravel in at the back and just covered it. No problems there, but the front started to become a problem, so having a mad moment I mixed everything up and added JBL AB. It created a hell of a mess, but it did settle. And no problems since, and it actually looks a lot better. I'm still desperate to change it though
AS is like clay pellets I think. Azoo make a knock off version which I got cheap from AE.
Sorry Mark, the 4ft is a mess. A real mess Its only ever had swords and densa in it until recently - Clive sent me half a kilo of crypts and other plants I'm getting rid of it in the new year and starting fresh with a 3x2x2' opti-white.
I did look for some pics to try and show the substrate...
Are you sure that it has definitely gone anaerobic? As in smell of rotten eggs/pH swings for no other apparent reason? I ask as you sometimes get gas bubble forming even in a healthy substrate, sometimes they are nitrogen/CO2 bubbles given off during aerobic bacterial activity and are nothing to worry about, you just see them more in finer substrates as they trap them more.
I've got sand in all my tanks and have never had any problem at all. I don't stir up the sand but simply vacuum any debris off the surface. I've have never seen any gas bubbles coming out of the substrate at all and not all my tanks are planted so it can't be roots keeping the substrate oxygenated. I think that unless the sand caps with high levels of organics (or carbonates perhaps?) and seals up the lower levels then surely there would be some water movement throughout the substrate. I do use thinner sand amounts on unplanted tanks, but still an inch or so usually.