First post long time reader, hopefully I can obtain some guidance from the board with what to do next with a issue Im having. I have attached pics. I recently started my first indoor grow, using a grow cabinet. The cab came with 3 100watt cfls and a vegetative shelf that allowed the seedlings to be closer to the light. So things started just fine the first pic is from nov 18th, of the biggest seedling. I only usually take one pic because the camera on my phone isnt that great. After about 2 weeks I decided to switch to an 180 watt led. That is shown in the second pic. Growth was still good and green. However about 2 days after adding the led, I noticed a little yellowing on the tops. So I dropped the shelf (they were about 6-8 inches from light on shelf, with shelf they are about 18-20"), and changed the water.I forgot to grab ater from walmart and only had 4 gallons for my 5 gallon reservoir. So i decided to add a lil tap water, because was rushing to work. I didnt check my box for about 36 hours because of work and family and when i came back the last pic is what i saw. After asking around a bit and google searches I ph’ed my water and came with 7.6 :(. So in addition to some light burn i may have a nutrient lockout condition. I have already flushed and im checking the ph level nightly now and it hover between 6-6.5. turns out local tap water has a ph level of damn near 8. Basically after about a week i havent seen any change so im wondering are they finished? Any advice going forward would be helpful. Im wondering if i should trim the yellow and dead leaves and hope they grow back? 
It’s hard to say with so many different things going on, but I wouldn’t be surprised if it might mostly have been that the EC/TDS/PPM was actually too high causing nute burn, and even before you added tap water and then added insult to injury throwing the pH way out’a wack. To me it mostly looks like nute burn, likely from not adding more distilled water soon enough. The LED’s may have contributed to the burning tips of the leaves, but it looks very much like nute burn, especially since the top or closest part of the plant to the lights is not necessarily the most burnt part.
This means you also likely lost a lot of root mass as well, the roots will burn as well as the leaves if the EC/PPM gets too high.
Remove all the dead burnt plant tissue. If the green is still flexible and soft and doesn’t crunch like it’s dried out and kind of salty/crispy(high nutrient salts built up in the plant tissue), then that part is probably still alive and if the core of the plant is still alive it should recover. However it took a big hit and it is going to take time to rebuild root mass and then it will be able to start rebuilding more vegetative mass, so be patient and treat it gently as it recovers.