This has come up before.. I think Tom Barr was even suggesting to use a substrate sold for freshwater and brackish planted tanks.. Onyx sand, but its a black substrate. I think you could use the Miracle Mud with some good results, but you would need to supplement with fertilizers most likely if you did go with seagrass. I've been using Jobes fert sticks from places like Home Depot.. the green houseplants ones though.. the flowering version is sky high in phosphates. I covered the mud level with regular aragonite from CaribSea.
I tried DIY yeast CO2 reactor for a week or two, but that was before plant density was really high. I didnt notice a big impact then. With marine, I'm a little nervous about playing too much with pH issues. If I bubble CO2 into the tank there is the danger of creating acidic water. If I bubble just air, it should aerate sufficiently to bring in both CO2 and O2 (and everything else in air) so that it tends towards a natural pH level, (dependent on the alk level in the tank). Seems like less chance of heartbreak from my point of view.
Flourish Excel, a liquid CO2 supplement for freshwater, didnt really have any spectacular effects. Plus, its got some algae-negative qualities, so I dont know if I'd subject macro to it without testing a little bit of macro tissue first.
Any other additives.. water column dosing of fertilizer.. mainly KNO3 for nitrogen and a smidge of KPO4 for phosphate. They do need a little to grow. That was the start of this thread actually.. I overdosed!
Only other major contributor is PMDD.. another freshwater planted throwback. Thats to try to cover all my bases with micronutrients - especially magnesium, calcium and iron.
>Sarah