I am not very experienced grower, I am more of a theoretical! But in this situation I would get a little air pump + liquid silicon and Ph down solution. As a tap water mainly has high Ph level, so, it has got to be reduced for flush, you will need to run your air pump in your water for a few hours. So that will bring Ph level down to neutral which is 7 (+-0.2). Than flush with this water, 3-5 litters at the time so it doesn't drown, wait until all runoff comes out and check Ph level as well as PPM. Than do it again and again, until your runoff Ph gets to around 7 and PPM to 100-200. Let your plant drain for few hours away from your extreme lights. Oxidize water (1 x your pot size) with you air pump same way you did for your flush, until it gets to neutral Ph level. Than add your nutrient mix (no more then 1/4 of the recommended dose), most of the fertilizers are bring your Ph level down to extreme levels, this is why you will need to add liquid silicone to raise Ph up, (also liquid silicon has some very good healing effects on plant so add it in any case). Check your Ph level again, your goal is Ph 6.6 (this is for soil, google recommended Ph for your grow medium). All you need now is adjust your Ph with liquid silicon, or reduce with Ph down solution but be careful as this solution is 81% phosphoric acid and it can burn you skin. Also add 1 drop at the time than mix it well and check Ph, do it that way until you get to Ph level you need. Than water with you nutrient solution, wait until all runoff goes out, and it's ready to go back under your light. Hope that helps.
Also important, when you water with your nutrients make sure not to get any water on your leaves, as it will leave some brow spots on the leaves!