I saw this exact sight in my own flood tray last night. I found out they don't like bleach water at all
Tonight is drench night, they all get an Azamax drench. If that doesn't work I'll just kill everything in my soil, including the beneficial life, with some neem. Die fuckers die! Then it is a diatomaceious earth powdering up top to keep the survivors from escaping. I've been fighting gnats since I started growing in '09, and the longest I can get rid of them is for a season or two until they go back in full force outside, and some how only hang out in my indoor crop, never the outdoor crop. Caterpillars have been my only enemy outside, and green leaf mining mini-crickets, fuckers, but Thuricide has been murder on the caterpillars, they DO NOT survive that shit.
I might even kill all the adult fungus gnats out of spite with a doctor doom bomb. Talk about "Knock-Down" power. I've been using that word a lot today... knock-down power, haha.