sorry i havent bee on in a bit..
it should be ok. i ussually wash off any excess soap on any rocks or dirt just in case..
i use a spray bottle so it foams up a bit. then i rinse the bottle well and put fresh water and respray so the soap isnt on the surface..
if you have rocks it should not be a big deal. just rinse the top of the rocks with fresh water ant the gnats should go away..
i did it in my gecko terraniums with them in it and they do ok..