I used some stuff called gnat nix and it worked awesome. Took about 2 weeks for them to all dissappear. Took 1.5 bags to do 9-5 gallon buckets. Stuff was under 15$ a bag and it is organic.
Do not use the sand infused with calcium carbonate. It's all but impossible to fuck up the pH in soil, thats one sure way to do it! Calcium Carbonate = Lime.