Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
I think greensand would work, and would be safe. It is a slow release sort of material. I have no idea how much, or how long it lasts. Probably: Do not use very much. A few tablespoons per bag of soil? When the plants start showing some sign they need more you could easily add more to the substrate in any of several ways.
Whether to mix it with the soil or not, I also do not know. I would err on the side of 'less water column contact' and dust a little on the bottom of the tank before adding the soil.
Or, if you want to make a richer mix of soil, use perhaps several handfuls of the soil, and mix it with all the slow release fertilizers of this sort. Put this in first, it may not be enough to cover the bottom, or maybe just barely, then cover it with the rest of the soil.
How to add more to an established tank: (These methods will work with just about any slow release fertilizer)
1) Poke a piece of PVC pipe into the substrate, working it in so you remove the substrate in that spot. Then drop perhaps a teaspoon or less in through the pipe. When you remove the pipe the soil will collapse in the hole and cover the greensand. Do this in several locations.
2) Freeze perhaps a teaspoon or less of greensand in a small ice cube. Press the ice cube deep into the substrate. Do several of these.
3) Put some greensand into a medicine capsule, size 00 (pretty big) and press lots of these into the substrate.
4) Drain the tank pretty low and make a hole in the substrate by hand, drop in a small amount and refill the hole. Do lots of these, then refill the tank.
5) Roll some greensand into a clay ball and put these deep in the substrate.
When used in the vegetable garden typical application is 50-100 lbs per 1000 sq ft., once per growing season.