Re: Rock Dust or other alt additives.
I don't see why adding a little rock dust, or worm castings, or other organic sources of fertilizer in small amounts would be a bad thing. I'm certainly not a frog expert but I can't imagine it being harmful to them - and with a decent layer of leaf litter, they wouldn't be coming into contact with it much anyway.
The one issue you might run into is plants growing too well. More available nutrients means good growth, but any plant in the tank that's prone to growing rampant (and lots of beloved plants in both vivaria and aquaria are considered 'noxious weeds' elsewhere) might take over. It just means more work for you to keep stuff pruned.