The gnat/fly problems can also be helped by using sticky traps. Greenhouse suppliers sell a goo that you can paint on plastic strips and just stick in the soil or on a wall. I seem to recall that yellow was a recommended color for the sticks. Kind of a DIY fly paper...only a shitload stickier. Works great on the bugs, just don't let it touch your plants.
Had a friend that used an old microwave out of doors to re-generate used carbon from his water filtration system. Now that was funky. As cheap as carbon is it seemed counterproductive (using all of that juice to salvage some burnt coconut husks) but that's just how he rolls.
I haven't searched yet but there might also be a beneficial insect predator available to control the gnat/fly problems. How about lady bugs loose in the grow room?
Just a thought.