Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
When I was running low tech tanks with full fish load (meaning fish food was one source of plant nutrients) I found they needed potassium and iron first, then started Excel.
I would not double the dose of Comprehensive. I would start a low dose of macros, depending on how much fish food you give your tanks.
Lots of fish food = lots of N and P already getting added, so skip these. Just add K.
Very little fish food = probably not much N or P, so add N, P and K.
As for iron, you might find that as the plants start growing better they will need more iron than is in Comprehensive, so you might want to add iron.