There is widespread belief that if one uses deep soil more than a few inches deep, it will result in an anaerobic substrate & create harmful gases such as hydrogen sulfide. Many of these fears are over stated in the aquarium hobby and tend to be overly exaggerated in many cases.
The reality is that it principally depends on whether the soil is rich in materials available for decomposition. Soil that is too rich in organics (such as compost) can get anaerobic quickly with little depth as active decomposition consumes all oxygen. However, choosing a soil with highly degraded organics and lower organic content as a whole (only 5-15% organic content) allows us to have very deep soil layers without issues. Hence, soil selection determines how deep we can go - compost should be avoided or used only in thin layers (an inch or less) near the surface (where there is more access to oxygen). Low organic substrates, or large grain sized substrates (such as aquasoil) can be layered thickly without issues.
MTS (mineralized top soil) seeks to break down these organics before utilizing the soil in an aquarium. This approach produces a much more stable substrate.
Most soils do become slightly anaerobic with depth. This actually has some advantages - iron is reduced to Fe 2+ format and more easily uptake by plants for example. Whether or not anaerobic soil is harmful is dependent on how deeply anaerobic it is and whether the soil composition have elements that become harmful when the substrate is deeply anaerobic (see chart at top of page). A soil that has very low sulphate content will not be able to produce hydrogen sulphite gas in harmful quantities easily for example. Most of the bubbles one see in the substrate are actually CO2 (most commonly) rather than hydrogen sulphite. Indeed all the oxygen in the substrate must first be consumed and reduced to CO2, before conditions can become anaerobic at all. So for folks constantly poking their substrate because they see bubbles - this is a most unnecessary action, most of the time it is harmless.
Also the fact that a substrate does not go anaerobic doesn't mean that it can't go bad. Again, heavy organic content soils that are too labile can burn delicate plant roots - even if the substrate layer doesn't go deep enough to be deeply anaerobic.
In short - not all anaerobic conditions are bad and the fact that a substrate doesn't go anaerobic doesn't mean that everything is fine. The two things can be separate.
In addition to this, aquatic plants channel oxygen down to their root zones - this generally prevents well planted areas from going deeply anaerobic as well. Plants aerate the substrate zone they are planted in and maintain the health of the substrate.
There is limited advantage to having a super deep soil layer (its a mess to clear as well), we can fill the deep portions of the substrate with inert coarse material instead. Crushed lava rock or baked clay chips are both popular choices as filler material.
In my tank above, I use clay chips as a filler in the bottom layer, a few inches of raw soil, capped with a few inches of aquasoil. The thick aquasoil layer is so that less raw soil is pulled up during replanting. The total substrate depth is 8 inches which gives a steep slope in my aquascape (see below).
As a general check to see if your substrate is healthy or not - look at the root growth of smaller, delicate plant species. Large robust plants such as lotuses and swords can tolerate worse substrate conditions than smaller delicate plants so they are not good indicators of substrate health. If the roots are blackened or if top shoots of the plants are always healthy, but there is always melting near the substrate - this can hint at substrate problems.
General guidelines for soil thickness:
- For aquasoil, 3 to 4 inches if you have deep rooting plants and want to grow them quickly or to maximum size. Smaller carpeting plants can grow well with just an inch of soil. Layering aquasoil thicker than this does not carry any risk - but it is probably wiser to use inert substrate for convenience of management and to save cost.
- For raw soils, if you do not know the composition of your soil, erring on the side of less is a smart move. An inch below sand is generally safe for most raw soils. If you can select a low organic content soil, then layering it thick is harmless as well. I have raw soil layers 4 to 5 inches thick in some of my tanks.
- Sand caps should be thin rather than thick; this gives plants easier access to the soil layer. I have caps as thin as a quarter of an inch.
Many technical hardscapes stack aquasoil high in the back for convenience to get an elevated horizon line. The coarse grain and highly degraded organic content of aquasoils prevent compaction or severely anaerobic conditions