Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Best to tear down the whole tank, sift the blended substrate and see if you can separate sand from Amazonia, but I think it will not be worth it.
Buy new sand, or whatever other substrate you want.
When you do this your tank will lose whatever bacteria are on the substrate, perhaps as much as 25% of all the bacteria in the system, and the plants will not be up to removing ammonia etc for a little while. Add a bottle of Nitrospira as you are refilling the tank to boost the bacteria population.
If you want to try a partial job, then practice with your gravel vac, and see if it will remove the Amazonia without clogging too much. See what it will do where the sand and Amazonia have blended. If this will work, then get the sand ready (rinse well).
Remove as much as you can of the Amazonia, and sand.
Put the hew sand in a bottle with a narrow opening, such as a soda bottle (perhaps 1 liter or so, a tapering neck is excellent. Something like Jarritos or Crystal Geyser flavored, carbonated drinks). Put the bottle of sand in the tank, turn it upside down and allow the sand to fall out into the tank where you want it. As the water rushes it to take the place of the sand it will tend to keep the remaining cloudiness in the bottle. Not 100%, but this is a very good way to add sand with a minimum of clouding.
As above, I would add a bottle of Nitrospira to replace the bacteria you have removed with the substrate.