I am keeping 3 adult vinegaroons in a 20 gallon with dividers. Lately i noticed a substantial amount of these flies all over the place in there. They are smaller than fruit flies and have black bodies. I check on my vinnies and they are fine but i do wonder what these flies are. Im going to change their substrate asap but if the flies are harmless i dont see a point. I know a few of you guy have seen this. So give me your input, what are they and what do i do?