You'll need a meter at some point.....I'd add SMALL amounts of nutes slowly....you don't have to change the whole res.....sounds like they need more dinner but watch them closely since you can't measure ppms. (by small amounts....I mean go to 1/3 strength nutes from 1/5 strength and watch them closely....if that's good for 4-5 days, then go to 1/2 strength...) check ph often....(as you have been doing ) a sudden jump in ph can signal the first signs of nute burn.