SOS Save Our Seedlings

My Master Kush has been lagging behind since the beginning. Then the cotyledon started turning brown and now it looks to be spreading, the tips of the real leaves are showing signs of yellowing
when i tested my water 3 weeks ago a was consistently getting 90-110ppm, well i tested it again yesterday and its up to 200! im pretty sure the culprit is the temp rising oustside, rasing the water temp, therefore rasing the solubility of whats already suspended in it. i cant do an RO right now my budget is blown, also why i cant replace the master kush.
so do you guys think the waters the issue? the MK is the only one having issues so is this just a sensitive plant suffing some nute burn? im thinking about flushing the cube with bottled water. is that a good idea? sould i use a flushing solution or just clean water?

pic #1 is a day or two ago, the rest are today. all three in the first pic were started together.

What are your total ppms with nutes added? Heating pad being used? Temps?

No nutes yet, i used 7 drops of GH rapidstart in 1 gallon to soak my blocks prior to planting but its been nothing but the above mentioned tap water since. temps about 75-80

I would get a fan on them if not already in place and try to get your source water to about 100 ppm by cutting it with bottled water (or you could use straight distilled water if you have cal/mag on had). I'd then nute it up to about 200ppm and try to keep the rockwool moist but not flooded as fan drys it out. Flushing seems like a difficult prospect on such a small seedling so I would avoid it.