if your having buig problems ive noticed a layer of perlite on top is hard for them to break through.
it works almost as well as sand but doesnt have the effect of sand on your plants. i noticed the bottoms of my leaves starting to die while having the sand in the pots. more than likely from salt build up or something im no expert but it began once i added it. that or the gnats were eating at them,
either way i took out the sand and added perlite and havnt really noticed any gnats in the pots.
also i found the fucking hive of these bastards, i had a bag or soil in my bottom dresser that i had completely forgotton about until i decided to take the sand out of the pots. i was going to add a little more soil but when i opened the bag i seen hundreds of the bastards in there so i hurried and threw it out. FUCKING SOIL. cant wait to go hydro