I don't use anything currently in the dart viv. I think if its cycled with springtails you wouldn't have an issue with the poop. I don't see why you couldn't use wood ash.

I find in my whites tank I have to take the Broms out occasionally since the whites really make a mess down inside it. I figure the broms there get enough fertilizer and a good rinsing just refreshes them.

My Red Eyes tank I do nothing and the plants there are out of control so I never thought about any fertilizer there,