IMHO low light is limiting, but that is just me.
10 pound CO2 could run a 90 for 6 months?
Depends on your method of diffusion.
DIY reactors are very popular attached to discharge of canister filters.
Lighting depends on your plant selection.
PAR of 30 @ substrate would be low light, 50-60 medium, 100+ high I guess.
Do you have a PAR meter from that reef tank? Keep it, it will help.
Substrates I work on the cheap.
SafeTSorb(high CEC), Soil capped with BDBS, inert quartz 4mm size.
Several people here are really into Discus, I cannot offer advise on this one.
With plants come fert requirements.
Way different than your SW setup.
We like nitrates and phosphates, 20:2 respectively.