You could add some laterite...keep meaning to do that, but never do.
My plants do OK, I add ferts to the water every few weeks, plus planttabs in the gravel.
If you want something finer, but not quite play sand....i bought a few bags of the "A.P." sand (next to the gravel)...it's ~ 3/16" down to dust.
...ran it thru a piece of window screen, and used the big stuff...either by itself or mixed in w/ gravel. added the small stuff to my garden. rinse well, of course...