Sand Caps are a Crutch (Opinion)
I finally made a reddit account to share my thoughts and experience..sorry for the slightly provocative title lol.
Ive been doing tanks and ponds for 6 years, started with father fish tanks and now I mostly do invertebrate only tanks for fun..
I have grown to be very anti sand cap. I stick to what nature does. My tanks are now created with 2 inches of backyard topsoil and clay (30% clay) for CEC.
I do no sand cap at all. I get a thin layer of local pond mud as my "cap" along with a thick layer of local pond leaves. I mix in a super thin layer (less than a quarter inch) of gravel / sand / sticks exclusively for texture, making sure that my critters can directly access the pond mud and topsoil without having 2 inches of inert sand in the way.
People are afraid of soil leeching into the water but the sand is a harmful crutch that limits your invertebrates from enjoying the proper natural pond mud. It also reduces oxygen to the soil.
My tanks have never had an issue, I can keep tadpols and koi even.
The thin layer of pond silt binds to the sulfurics and acts as a thin cap, the biofilm assists and then leaf litter is a lightweight cap that allows critters and oxygen to directly interface the soil but also helps keep things settled down.
Even in areas with directly exposed soil and just a tiny bit of local pond flocculant, the water stays perfectly healthy and clear.
The sand cap came to be when they tried to idiot proof the natural tank setup but it does harm and is not necessary! Even direct topsoil exposure is FINE after a couple weeks or so but the pond silt lets you skip that waiting.
Thanks for reading..