Dolomite lime question

I have some questions hope you guys can help. I found out one of my clones has a magnesium problem so i checked my ph tap water with a drop tester kit and it came out to be 7 so then i checked the runoff and it came out to be 5!! Should I buy some dolomite lime powder to fix this problem? And if I add dolomite lime can I add it on top the soil since she's already planted? Can dolomite lime help to fix the magnesium problem?

I'm growing in FFOF outdoor and no nutrients since FFOF soil has enough already