They are wrong.
You do not have nute burn.
You have magnesium deficiency caused by too much calcium in the water you are using.
Not all bottled water has the calcium removed.
Only RO and distilled water have the calcium removed.
Rainwater is another source of calcium free water.
You have too much calcium.
Adding more magnesium is not the answer
as some would lead you to believe.
The only way to grow like a pro, is to use RO.
When you finally switch to RO
you will never look back.
Get an RO machine from ebay
and hook it up to a RO only rez,
so you will have a supply
when you need it.
Dirt or hydro,
RO will pay back big time.
.