I have used it before in my four foot tank. It was mediumly stocked and the nitrates were always at zero which presented problems. These tanks can be stocked quite high, often higher than is recommended with filtered tanks. Basically the organic matter in the substrate decomposes to produce co2 which the plants use, they take up waste produced by the fish. The soil contains micro nutrients that are not necessarily available in the water column, the fish are fed a bit more than they can eat and the excess food decomposes to release co2 and other nutrients the plants need. Plants prefer ammonia over nitrates, I think they have different charges and the plants have to convert nitrate ions into a positive or neutral ion, not too sure though sorry. Filters will compete with the plants for the ammonia which is why they are not needed. These tanks often become deficient in potassium (K). This can be overcome by putting a layer of dynamic lifter at the bottom of the substrate or by dosing potassium nitrate (KNO3). This can also be dosed if your finding you can't supply the plants with enough nitrate, you should have at least 5ppm nitrates. In the start a lot of quick growing stems are needed and as the heavy root feeders become established the stems can be removed and replaced by heavier root feeders such as crypts, lotus and swords. You want to keep the substrate slightly alkaline, hence the shell grit, to ensure hard water nutrients are available. If the substrate becomes acidic it can lead to iron toxicity. The water will naturally become neutral because of the various biological process occurring and if your ph is rising or dropping it usually means your tank is unbalanced or an object in the tank is affecting your ph.
These tanks naturally grow large amounts of micro organisms and make great fry tanks, I'm going to be spawning in this one soon.
You want to soak your substrate before using it if possible, especially if it comes from a bag (potting soil) it could release ammonia on set up. I add the zeolite to avoid this as this soil had aquasoil which releases ammonia when new and I didn't soak it. Zeolite also has a high CEC which is supposed to be good for plants, I think they can access the nutrients it takes up.
Was this helpful? It is in no real order and I wrote everything down as I thought of it lol.
Here is my 4 foot tank in its prime, it is not looking to good now, I’m in the process of going high tech because some of the plants I’m growing don’t go to well with this method. It was great in its prime though and only has algae because I’ve unbalanced the lights, plants and CO2.