Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
There are 2 ways.
1) Take most things out, including the water and add the EC. Put a plate or plastic bag over the substrate and refill slowly so the water runs over the plate and seeps into the soil.
The whole project could be done at one time.
2) Use a piece of pipe perhaps 1" or a bit larger and longer than the depth of the water. Aim the pipe where you want to EC to land, touching the substrate, and put the EC in at the top of the pipe. Gently lift the pipe so the EC falls out without getting too high up in the water. Stop when the water gets a bit cloudy. It might take several days to finish the job.
If the material is really clean, then you could also try this:
Use a small container that will get in between the plants pretty easily (Maybe a yogurt container) and add the EC with that. Make sure it is wet, gently guide it down as close to the bottom as you can, then pour it out slowly. If the new substrate has any tendency to cloud the water this is not a good way to do it, though.