typical deficiency but only one in a 6 plant system

So I have some sort of deficiency going on here. I was wondering if someone could diagnose. I know it looks like a typical magnesium or maybe nitrogen deficiency? However its only on this one plant in 6 plant dwc system. I had fungus gnats and put a bit more gnatrol in my resevoir for the last three days than I should have. All the gnats seem to be gone and im about to change out all the water but I was wondering how you guys would go about treating this one plant in a system with 5 other healthy plants. Maybe it didnt like the overdose of gnatrol and when I change out the water it will return to normal... I've also been putting in much less nutes than I should so maybe its not getting what it should? Its also the only plant that roots hasnt reached the water yet so maybe the gnats slowed down root growth a lot for how big its gotten to get the nutes it needs?

They are all about a foot now and after changing out the water im going to give them the "late growth" amount of nutes from gen hydro which is 10ml grow / 10ml micro / 5ml bloom.

Any other ideas or should changing out the water and giving new nutes be the only thing to do at this point?

Thanks for any ideas and replies!

The only way I can think of treating the one plant would be a foliar feed. are they all the same strain?

no... three are sativas like this one and three are indica...

this one is skywalker with I think cross with G13. You think this is more magnesium problem or nitrogen or something else? I think switching out the water should help since there was a lot of gnatrol but not sure what to give it in terms of foliar spray

I forgot to mention on pic unnamed.jpg on the bottom leaf you can see its really brown in between the pedals themself and the other pic you can see other / healthier leafs wilted down a bit

foliar feed with a weak grow solution and change the rez. it will ride out just fine

cool cool, appreciate it! Thanks again Velvet Elvis!