So I did a little digging around and found this; https://aquariumscience.org/index.php/8-5-under-gravel-filters/
Which is not exactly the one I have. The I am using is an old model which is white but basically the same thing. Agreed that I have considered that the substrate may migrate under the plate. A couple solutions offered have been nylon window screening or the black nylon landscaping fabric. Or to just leave it as is. As for the potting soil I have been using Miracle Grow and putting it in a bucket and flooding it to try and strain out the large pieces of wood and debris. The peat moss, the label says it's in there but in the small tank I am using as a nursery for my tank plants the water is crystal clear.
Anyways this 36gl planted tank is a project and as I am retired I have plenty of time to try things and not be ashamed of making mistakes and admitting them, then starting over. One of the things I am learning is drift wood and tannin's. I could live with a stained color but this is also looking hazy, cloudy. Not milky cloudy.
Because of the staining that I can't get rid of and the poor performance my plants are now experiencing I am contemplating breaking this all down and starting over.
Picture 1,2,5 are the 36gl Planted Bow. it has 5 Gourami and a doz. Amano shrimp and god only knows how many red and blue shrimp. The 3rd picture is the 29gl high which is the wifes and contains Tetra Glo, a Glo Shark, a Golden AE and 2 dwarf Gourami and also shrimp. The 4th picture is the Fusion Micro 30L That was acquired used about 6 weeks ago and presently has 2 CAE and a Red Tail shark. These two CAE came out of my 36gl because they kept destroying the plants.