4-5 inches. Some spots are less but I keep the rooted plants in deep substrate. I also collect my own plants and some of the detritus/mulm comes with the plants. This whitish dusty mulm is mainly CaCO3 from the Halimeda beds with some organic matter.
You can enrich agar with about anything. I have KNO3, KH2PO4, K2SO4, Trace mixes(These are WAY cheaper than Marine traces mixes they sell and are pretty much the same thing, some are missing things like B, Sr though but these are not large issues if you do water changes and raise plants/macro's).
You can add sucrose as well if the plant has very little photosynthetic tissue after trimming etc to give it an extra boost to get started. This can help some slow growers. Boron can be bought at a drug store, boric acid. Also good for keeping ants out of the house.
Careful not to compare Reef(critter) sup's vs macro and plant sups.
I've alway held the notion not to worry about the algae I don't like, but rather focus on the plant or macro I do like. If you give the macro or plant good conditions for healthy growth, you don't have other nuisance algae problems.
And growing nice healthy plants/macro's is the goal, not fighting nuisance algae. So I focus on the needs of what I keep. I think this focus works best in the long run.
Plants have always preformed at higher uptakes than algae in all the FW scrubber vs plant scrubber set ups I've done and with less light intensity to do the job. I think the macro algae and refuge's do better than the turf scrubbers watt for watt and then you are left with nice plants rather than a turf. Some feed both or either to their fish but few folks can get $ for turf algae. I have not measured much in my tanks concerning this in SW but I'll get around to it. I use the plants for the tank itself rather some slave for nutrient removal for a reef. A plant can be used for any and every tank type pretty much.
I am surprised that I have little nuisance algae with my dosing routine. But the growth is good and healthy. When the plants/macro's don;'t grow good, the tank gets nuisance algae. Therefore the focus should be on optimizing the plant/macro's needs, not worrying about limiting this or that nutrient. That's my opinion anyway. It works and has for many years. Happy plant= happy fish = happy hobbyist.
Regards,
Tom Barr