Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
8x is just fine.
I aim for 10x per the labels, and I think I am lucky to get about the same as you.
Over time an aquarium will reach a balance of nutrients in the soil getting used by the plants and getting replaced by the decomposer organisms working on fish poop, fallen food, dead plant parts and so on. This takes time to develop. Water column dosing also gets fertilizers into the substrate. If the substrate has a high cationic exchange capacity there can be quite a lot of activity going on. If the substrate is more like a sand or gravel the water flows through faster, because of the larger space between the particles, but the fertilizer flows through with the water because those coarser materials do not have significant CEC. Eventually the organic matter (mulm) will get broken down small enough to have significant CEC, so even in pure gravel substrates there can be a reserve of fertilizer.
In the case of a high CEC substrate with a sand cap I see no problem at all with the fertilizers in the water working their way deeper into the substrate. The sand is little or no barrier to that. As I said above, it just takes time. The plant growing substrate will not become a fertilizer-rich substrate overnight, but it will happen.
Starting with some fertilizer in the lower part of the substrate can get the plants growing well without the water column dosing that might contribute to algae until the whole system is getting established.