Rusty spots, leaves dieing (pics)

These are Super Girl from clone. Under 24/0 T5 Flouro for veg. They are getting full strength Lucas formula with GH nutes for 3 weeks no problems. They are in DWC buckets.

So a few days ago I noticed small rusty spots on some of the leaves on one plant. The next day the spots multiplied a great deal! With some leaves becoming covered and dieing.

The pH was very low, around 4 I think. The roots look ok... just slightly brown. I dumped the nutes and gave them pH balanced tap water with 1/2tsp gal of epsom (i know epsom isnt a miracle cure i was just hoping i guess)

So after doing some research I'm thing the damage is caused by fungus gnats in my root systems. My soil grow brought the FGs into my growroom, but i hadnt noticed any in the veg room DWCs. I examined some of the original acidic nutes under a pocket scope and found several silvery swimmers in there.

I've treated my storage res and DWC resivors with Mosquito Dunks, active ingredient bassilus thuringiensis two days ago. I seems like the spotting might be slowing down or stopping. I've also resumed full strength Lucas formula. Although the damage may be slowing stopping, it is much worse than these pics, which were taken 2 days after noicing the problem.

My questions are:
1. Can anyone confirm that this damage looks like fungus gnat damage?
2. Does anyone have experience with using these mosquito dunks in DWC systems for fungus gnats? How long does it usually take?

Also, do fungus gnats lower the pH of the solution? I figured that might be causing a lockout?

Never seen fungus gnats do that. It looks more like a P deficency or a PH problem or both. Whats the NPK of your nutes? The recomended NPK for veg is 10-10-10 and for flower its 10-30-20. The recomended PH for hydro is 5.2-5.8. If its around 4.0 its way too alkaline for your plants. You need to keep the PH and NPK close to the recomended range and dont overfeed. It will make a big difference. Good Luck!

I adjusted he ph yesterday back up to 5.5 and today they are back around 4 again! The roots dont stink and look fairly healthy. Yeah, I think I was way off with the fungus gnat thing. I emptied each bucket to almost 1/4 than replace with higher pH tap water.

Anyone esle have any input? much appreciated!

ok for starters 4.0 ph isnt to alkaline its to acidic lmao . alkalinty being above neutral 7.0 .

i say keep your ph at about 6.3 trust me on this u will see alot of improvement , get your water +nutes , then add about 1 tblspoon for 3-4 litres this will raise the ph alot ok .but then u gota bring it back down to around 6.2 , the BS acts as a ph stabilizer .

looking at your problem looks exactly like the problem i got , with the rustylines down the leaves n the rusty spots . i believe it to be ph fluctuation , when u add your ph down i hope your not adding to much to quick u can really fuck up your plants .

just try this it might work .