Actually 1tbls per gallon is pretty much the standard recommended amount, anywhere between 1tsp and 1tbls and you should be fine, thats not the problem. Some strains have naturally purple stems, it could just be genetics. What strain you growing? Also, you could have burn from nitrogen and deficiency of mg, they aren't mutually exclusive, although with all the nutes you're using I would guess burn for the tips and genetics for the stems