Mushy cotyledons, exquisite..
Like Sedge said, a few pics will help but also you might want to give us a complete rundown on the growing media, you also have alkaline water don't you? Might just be a bit of ph lockout if the larger leaves are effected also but i wouldn't worry too much, stressing about it will result in more compounding issues which will probably kill the plants. I've killed plants before by constantly googling and trying to diagnose problems when in reality i was just stoned as heck spinning around in circles chasing my tail lol