Critical royal queen seed

Topped this plant and LST also. Still at ph 6.5 of rain water and spring water, however the leaves are crumbling and turning brown, I’ve move the light up more so it’s about 45cm away now. I’ve gel it a ml of baby bio and tomato plant feed per gallon of water, I’m unsure if this is to little nutrients for but I’d rather under feed than over feed. Please feel free to leave any advise; this is my first time growing so I need all the help I can get☺️💗