i use pond water mostly.
i also u 15-30-15 MG.
ive never had a problem with MG.
i use it almost every week at full strength. 1 gallon per plant. plus i have organic ferts under my mulch around the stalk. 6-3-3 and some other organic ferts... ive had no problems with burn with this combo of fertz.

the purple in the stems means they are hungry.

6 small scoops equals full strength with MG for 1 gallon. plus u have run off if u have drainage holes in your pots.