I've got a 30-gallon tank with about 5 cm (~ 2 inches) of natural gravel. It's a mix of two parts fine and one part coarse, where the fine is about 2mm and the coarse is up to 1cm. I'm working on the water condition and light to allow better growth of my surviving plants (crypts, micro sword, banana plant and dwarf sag) and some new ones (maybe parrot feather, water sprite, alternanthera reineckii, pennywort, java fern and another sword).
Additional substrate may be needed and gravel doesn't seem to be the solution that will work best for my plants. If the plants aren't doing well, my fish will suffer. I want to add another 2 or 3 cm on top of my existing substrate. I'm hoping that it will mix as it settles on its own into the gravel and is vacuumed lightly every two weeks. If I only had to worry about the plants, I don't think this would be a big issue. However, I have 2 corys, 4 otos, 5 lemon tetras, 6 pristella tetras and 4 neon tetras. Has anyone had experience adding substrate to a tank that has fish in it? Any suggestions?