A long time ago, I thought your argument was a good one ... so I bought a dry Iron Sulfate with the thought that it might make a good substrate additive when setting up a new aquarium.

So I did several tests. I poured a thin layer into the bottom of a new 55 gallon tank and then covered that with ordinary "red" colored aquarium gravel. In a second tank, I poured the same thin layer and covered it with a similar layer of red gravel and then a layer of sand. And in the third tank, I simple used the red aquarium gravel. I then dosed all three tanks using the EI method.

I was expecting some improvement in at least one of the tanks using the Iron Sulfate in the substrate.

I was concerned that the Iron Sulfate would leach out of the tank that was just covered by gravel.

I could detect no visible difference in the health of the plants between the tank with Iron Sulfate and Red Gravel and the tank that simply had the red gravel.

I could detect no visible difference in the growth rate of the tank with the sand covering the iron sulfate and the red gravel, however, the tank was susceptible to a black slime type bacterial algae that was difficult to keep in check - that I have never had in any of my other tanks.

That is not a perfectly controlled experiment ... and it is only an experiment with relatively hard water with GH/KH around 11 ... but it gave me enough information to decide that I did not think it was necessary or appropriate to sell ordinary Iron Sulfate ...

Greg Watson
The "other Greg" in the context of this thread <grin> ...