1 and 1/2 inches of dirt and about an inch of cap has worked good for me
From what I have seen, dirt looses its kick after a couple years, but it still continues to grow plants great, albeit, at a slightly lower rate. But the thing about dirt is it continues to use nutrients from fish waste so it never really stops being beneficial to plants. I have seen tanks set up well over five years with the same dirt, and the growth has barely slowed. I don't know if there is anything that will make the dirt last longer, but red clay is extremely beneficial to add to the dirt to give the plants iron.Great thanks, any idea how long the dirt will be beneficial to the plants for? and is there anything else I could add to the dirt to improve the quality of it and make it last longer?