And here comes the different answers.
Topping off the rez with or without nutes isnt a hard nailed question for me,if your running a low ec or ppm then adding nute water most likely isnt going to cause any issues but................yep,there is allways a but.
If your running the nutes at the plants max or even close to the max of what the strain can handel then adding nutes can cause burn & or lock out,i'll explain.
There are roughly 16 macro & micro nutes in our fertilizers,some more some less,the total sum of all these different nutes is what makes up the ec or ppm.
Now lets say your feeding the plant 1,000 ppm in a fresh rez,when you check it 24 hours later the water is down & your ppm is down as well,this obviously means the plant drank water & ate nutrients,you top off the rez & re-check ppm & its at 800 ppm,so now you want to add 200 ppm worth of nutes to bring it back up to the 1,000 ppm starting point.
Here's the fucked part,you know the plant ate 200 ppm worth of nutes but EXACTLY WHAT NUTRIENTS DID THE PLANT EAT ? Lets say the plant ate mostly nitrogen & no P or K or any micro nutes,those levels stayed the same & the missing 200 ppm was nitrogen only.
Now when you top off with nutes in the water you brought the ppm up & increased levels of nutrients that the plant is not feeding on or using very little of,now another 24 hours later you see the same thing where water level is down & ppm is at 800 again.
Once again exactly what nutes make up the missing ppm,now you top off a 2nd time with nute water & what happens if the plant is still only eating nitrogen from the total npk & no other nutes ?
Once again those levels rise,this question or dilema im talking about is the exact reason so many newbs cant figure out whats wrong with the grow,they check the ppm & everybody tells them the low ppm the plants get in a fresh rez wont burn plants so it cant be nute burn or lockout.
Adding nutes to top off water is the fastest way for a new hydro grower to get into all kinds of problems because many of the nutrients end up at seriously over loaded levels & locks everything up & cooks the plant.
I would reccomend on your 1st grow to just use plain water & change your rez more often,if you dump & refill the rez once a week its the safest way to provide the plants with everything they need.
When you get a few grows under your belt & know exactly what the plants are supposed to look like at all stages of growth then start adding nutes,until then play it safe,topping off with plain water will not deprive the plant from anything it needs to be healthy,on the other hand topping off with nute water puts you at serious risk of nute burn or lock out,or both.