I would think you could also maybe try luring them out and killing them. In my house when we get fruit flies (I'm guessing that's what drain flies are?) I get out a shallow bowl and fill it with apple cider vinegar and put in a drop of dishwasher detergent. The pungent smell lures them onto the liquid and the soap keeps them there. It makes a stink but it does the trick. Maybe keep a bowl somewhere nearby the tank and see what happens, on top of cleaning the tank?