Sweet potato, sand and D earth for fungus gnats?

I killed 5 little gnats today and im getting pissed. right after I did a flush this shit happens. I only have 1 sweet potato, fine sand and diatomaceous earth. I just put the potato in slices on the soil and some in the growroom for tonight. around noon tomorrow I will put 2 inchs of sand and a sprinkle of earth on. Will this be ok or am I too late if theres adult flys around?

One leaf has a little hole the size of this ( * ) is that from them?

I think they might be gone. I used jumping spiders, potato and h202 in the water. I put an inch of sand on and that works.