Yep... deffinatly the yellow tips are the beginning of nute burn. So you've found the max nutes that plant can take. Those plants look nice though so just back off the dosage a bit and they'll be fine. Happy growing!
Well... I'm a soil grower. That's one problem you run into with ebb/flow. I've known growers who have set up a smaller system just for a touchy strain that they really wanted to keep. In the commercial grow we have lots of strains and the ones that are too sensitive are eliminated. We can't...
All strains have different nute requirements. So yes one might suck up way more nutes than another strain. The yellowing tips are generally no concern. When I'm dealing with a new strain I intentionally try and get the tips to yellow. That's when I think they have reached their max nute levels...
What kindd said is exactly right. Get clones of a friends plant with rock hard buds and grow them side by side and see. Strain makes most of the difference in hardness. Also you can cure loose buds and make them nice and hard...
PPMs always rise in reservoirs. Evaporation and built up salts (unclean res) make rises, plus break down of nutes. Topping with fresh water and cleaning is key.
I wouldn't worry too much about it. The first set of leaves have a tendancy to be funky. Generally though I don't fert until they're bigger than that, say at least 6 inches with several sets of leaves, so a little nute shock isn't out of the question, but 1/4 strength shouldn't hurt them. If it...
Just get it home ASAP without exposing it to intense sunlight or high or low temps. A small cooler works good just make sure it doesn't tip over by bracing it up right.
It does look like nute burn. I would trim off the burned leaves since it looks like you still have viable tops. Then I would flush with good water. Then no fert until you see new growth on top. Never give full strength fert right away. Start at half strength or less.