Only copper is harmful to inverts, everything else is fine. There is a lot of substrates to choose from like Flourite, Eco-complete, Organic Soil, and ADA aquasoil. A lot of them are high in CEC, which adsorbs nutrients from the water column for the roots to feed off. Iron is important, but it's not the only nutrient root feeders need. Your 3 major nutrients for root feeders is still your 3 main macros nitrogen, phosphorus, and potassium. I ran eco-complete for a year with my shrimp and fish with no ill effects. I also did organic soil with a sand cap with no ill effects. My next project is ADA aquasoil with power sand S, bacter 100, and tourmaline BC, but this stuff for my next project is super expensive.