Join Date: Mar 2007
Location: Hendersonville, TN
It all ends up getting mixed together eventually. May not to the extent of the mix that you received, but a mix all the same. This is due to several factors, like gravity, current, fish activity, vacuuming, moving things around in the tank, re-planting, etc. Adding thicker sand caps can be done, to an extent, but the thicker the sand cap, the higher the chances start becoming to get anaerobic issues in the substrate.
So, I would recommend using thin layer of kitty litter, a half inch of Aquasoil, and cap that by an inch and a half of black Tahitian moon sand or Eco. The kitty litter should stick to the bottom, and should not mix appreciably with the rest of the substrate. If there is mixing, which there will be over time, between the sand cap and the Aquasoil, at least it will all be the same color.
Or, you could just let the plants grow and carpet the substrate....not much use worrying about substrate mixing if there are plants covering it anyways. Bear in mind that you are worried about an occasional aesthetic issue....is it a huge problem? If so, than go with the option above. I think that all substrates can and will get messy with the accumulation of bacteria, mulm, dust, poop, etc. that will create a cloud in the water column. It is usually filtered away within an hour or two....or overnight in really heavy re-scapes.