Hey Growrook, you're doing a great job, overall!
If I had to guess, based on what I've seen and read, your fungus gnat problem is worse than you think. Those little f'ers can damage the roots which prevents the roots from uptaking the nutes it needs.
Fungus gnats love coco, and may have even been introduced to your growroom from a bad batch of coco you purchased.
You can get some Baccillus Thurengiensis (BT...and not sure if I spelled it exactly correct!) and add it to the nutes, which will kill the larvae, but not the adults. A Pyrethium fogger can help a lot, if you want to spend some money, and those No-Pest-Strips will also help.
Bingo. I agree completely, and even have a theory. Fungus gnats like... fungus. Unfortunately, when you have fungus gnats you often have fungus. Not only that, they
spread fungus. Bastards, right?
My guess is you may have burned them at some point (nute burn). When that happens, you see the damage on the leaves, but the roots are often just as affected if not more. Meaning, dead roots in the pot. You may have been overwatering some, too, or the pots were unable to dry fully in the middle due to straight coco as medium (see my other note below) which helps contribute to mold, fungus, rot... Just the sort of stuff that fungus gnats LOVE (other bugs like root aphids, too).
Like Mr. Krip said, BT. Think "Mosquito Dunks" (google if you're not familiar). Put them in your reservoir or watering can/bucket/whatever. One will do it, even half of one. I put it in a clean nylon sock, as they disentegrate into little chunks, but I only do that because I use the water with an active hydro system, too. Also, azamax works great. Or, the SNS 203 from Sierra Natural Sciences (a sponsor):
SNS 203 Soil Drench Spray | Sierra Natural Science
Follow the directions. I'd
consider getting rid of any plants that look really bad, so it doesn't continue to contribute to furthering or spreading the problem - the affected plants look pretty rough, in all honesty.
Also, consider things like "No Pests" strips by Hot Shot (google if you aren't familiar, I see Mr Krip mentioned that too). They work and are fairly safe as long as you don't spend more than 3 hours per day in your grow room, but read up to make sure you are comfortable with it first.
Also, there is a spray product called "Home Defense Max" that I spray outside the entrance to my grow room, as well as around the exterior of my home, etc. It forms a bit of an invisible barrier that bugs don't like to cross. I've literally seen bugs retreat as they approach the area I sprayed with it, so it does help some. For what it's worth, all of these things are part of my preventative maintenance approach now, as dealing with bugs... bites.
Oh, and if you have a utility sink nearby, dump a shot glass full of bleach down the drain. Make sure there is no standing water anywhere exposed - it's been proven that they can breed in a thimble full of water!
Finally, deal with the fungus. If you try to keep the affected plants, try adding an enzyme product like Hygrozyme to help detoriate and clean up dead rotten roots that are under the surface. Make sure you only water when the pots get light. With coco coir, the pots will get ridiculously "light" feeling when they are ready for watering. Basically, the best time to water is right before they start to wilt from lack of water. If you pick up a pot and think, whoa, that's crazy light for what I'd expect, time to water. If you're not sure... wait.
If you use something like Superroots "Airpots", or "Smart Pots" you can get by with straight coco coir as a medium, but if you are using a standard gardening pot, it might help to add perlite or something similar to help get more air in the medium which should also help prevent fungus, etc. (in the future).
Don't panic or get down - Mr Krip is right you are doing a good job so far. But, that's where I think your problem lies, too, personally.
Growrook!