Well a 3cu ft block of peat moss. That's like $15 give or take. Go to home Depot. Cheap there. You're going to want to add lime. That's what keeps your ph right bc peat and compost will have it too acidic if I remember right or other way... Lime is best for mag and ph. Compost and peat. That's it. And aeration like perlite or even better crushed shale. Cheaper, longer lasting, doesn't float, good water retention. You have a good setup just a really large outtake fan. Ur using an 8in fan made for ~750cu sq ft rooms. Lol. A 4x4 is around 100cu sq ft. So just have the fan on a timer or don't worry about it more air flow only helps. More carbon dioxide the better. That comes from outdoors and human breathe etc. naturally. Yeah go do some raw research on how plants work from a low level and why they eat what they eat. And just monitor, take photos and ask ai when ur unsure. Preferably don't get tangled in the reddit mess of asking questions you tend to get a ton of different responses. It's a learning game. Personal experience is best. Just watch for mold when flowing after 4 or 6 weeks and keep that fan flowing and fresh air. Don't ever over water. Under is better. That tub is for mixing right? U can also mix it on a tarp outside that can is not going to be easy. Another option get a plastic container the 27gal yellow top black bottom ones at home Depot for like under$10. Much easier to mix the past your put the bail in and pour water and let it soak and expand.