First off what conversion does your meter use? .5, .64 or .7. All I can say is maybe your water is screwed 300 ppm is high for tap water. mine comes out at 125 ppm. also why did you soak the cubes in 1/4 strength bloom? i could maybe see clones, but from seed soaking the cubes in bloom fert? obviously they are way past that but why do you do it?why soak em in bloom ferts? I would thouroughly flush pots with distilled or as close to 0 ppm water ph 5.8. then let them drink just that for 2 days, they may strech a bit, but will be fine. then EC @ 1.5. imo