First grow - auto 60 grapefruit

So, found a couple of yellow bits on my leaves a couple days ago which I thought was a nute deficiency so I gave her her first nute drink which didnt go down badly. Then I found a few fungus gnats over the last week, maybe 10 in all which were promptly squashed, from reading it seems like they come from overwatering and can be in your soil at purchase which is a bit of a bummer (and munching larvae can present nute deficiency synptoms). I didnt think i was overwstering as I felt quite tight on liquid for the size of the pot and always had some run-off with about 1L a week. Then again, rookie error #1 eh? Letting her dry out a few days now and will be adding some peroxide 3% @ 1:4 with water to get rid of the gnats, will add some nutes to that too for her second proper feed, hoping the root O2 boost from the peroxide and some nutes will do her some good. Started LST which seems to be going alright even if its a bit fiddly, not sure if im over doing it but ive spread her wide and got a few nice budsites appearing so dont think I'm doing her any harm and everything seems to be growing happy. Just need to make sure I'm feeding & watering her right now as shes growing a LOT at the moment, seems to be related to LST but I could be wrong, coupled with the fungus gnats and a need for nutes its a balance i'm gonna have to establish quickly this coming week. Was hoping to do this one organic but since finding the fungus gnats pretty sure the peroxide is going to ruin my organic soil cultures so will be sticking with mineral nutes at 1/4 - 1/2 dose for this one now. Going mineral nutes as im under the impression organic nutes wont do much after I peroxide the soil. She hit preflower too which I'm not sad about, looking forward to some mega growth this next couple of weeks :) Thoughts and suggestions anyone?