It's phosphorous. Since you still are in Veg(no PK succing buds) sth is wrong with pH. As yours is at 5,4 that's too low. I'm doing hydroponics /aeroponic myself and figured out that during veg not to go under 6 and during flowering from 6 to 5,6 max. Now plants are looking better especially no purple stems ore weird colored leafes. 2 weeks before you cut them I wouldn't mind them being colored,but since you are in Veg you should Def do sth.