i would like to know some opinions on gravel + laterite mix
If you are using true laterite, you must cover it with gravel. Laterite is simular to clay, and when you add water it turns into mud. It should be at the bottom of two or three inches of gravel or it will cloud your water and make a big mess.
Laterite is extemely high in iron, even more so than clay. It also has good CEC, (cation exchange capacity) which means it attracts positive ions, (minerals) from the water and holds them basicaly like a sponge. So whatever minerals are in your water or released into the substrate, it will absorb and hold them for the plants.
Before clay gravels were invented, laterite was the only choice other than peat or soil, and was very popular. Clay gravels like Eco complete and Flourite and others also contain iron and other minerals. Eco complete is fortified with specific minerals other than iron to make it "complete"
All these products work well and pretty much do the same thing. You can mix Eco complete with standard inert gravel but you don't want to dilute it by more than 50% or you really loose its benefit.
The reason clay and laterite was developed for the aquarium is because it is inert, non organic and will not decay and release organic compounds like soil or peat will. It will not release nitrogen or ammonia, only oxidized minerals.
