Hydro topping off advise required

Hi all hydro guys & gals.......... I require some info on this topic if possible.

Current situation
I'm Currently growing in x2 205 gt nft trays with a res size of 20 ltrs each.
Each tray has 1 plant in and as these plants are now in flower and there getting quite thirsty.
So in 1-2 days over a third of the res water/nutrient has been drunk which in turn is causing the ec nutrient strength to rise.
At the moment I'm topping off every day to keep the nutrient strength stable.

What I want to do is setup up a auto top up system on both trays.
I have all the necessary bits and bobs to do this but the question I want answering is...........?

If my header tank is full of ph water to top up the trays let's say I set my nutrient strength in both trays to say ec 1.5 as I top up the trays with the ph water from the header tank the nutrient strength becomes diluted over time so the ec level starts to drop. Now if this happens how do I know if I'm under feeding or just time to change the nft tray nutrients for fresh.?

Hope this makes sense to everyone.

Cheers

It makes plenty sense,underfeeding is very rarely an issue when topping off,stacking unused nutes causing overload of whatever nutes the plant is eating slower is usually the issue.

Lets use the 1.5 ec example you used,on monday you had a fresh rez with 1.5 ec,by wednesday you have topped the water off twice with plain water causing the ec reading to drop to 1.1 ec,where did the 0.4 ec go,thats an easy question to answer,we know the plants ate the missing nutrients.

Now for the hard part,what nutrients make up the missing 0.4 ec & in what ratios ?

Lets say the plant is currently using high N but very little P & K,if you refill the rez with full strength nute water to where the ec is brought back up to 1.5 there is a good chance you have now raised the levels of P&K high enough to throw the mix ratio off balance,worse case scenario causing nute lock while still having a low ec.

Lest say your plants max ec is 1.5 without burn or lock out,if the plants are not feeding in even amounts when you top off the rez with full strength nutes its very likely one of the many nutrients will become so high it causes burn or lock out,it is never advisable to refill the rez with full strength nutes,at the same time its impossible to know how much of what specific nute to add,so we guess.

You should allways err on the side of caution,i mix my rez at full strength & then every other refill i add 25% strength nutes,one day plain water with the next day 25% strength nute water.

This issue is why its in the plants best interest to do a rez change once a week,its the only way to be sure your plants are eating the best they can.

You can push a rez change to 2 weeks safely by using an oversized rez but it needs to be nearly twice the volume to assure absolute optimum ppm levels,all in all changing out the rez once a week is the best route.

It makes plenty sense,underfeeding is very rarely an issue when topping off,stacking unused nutes causing overload of whatever nutes the plant is eating slower is usually the issue.

Lets use the 1.5 ec example you used,on monday you had a fresh rez with 1.5 ec,by wednesday you have topped the water off twice with plain water causing the ec reading to drop to 1.1 ec,where did the 0.4 ec go,thats an easy question to answer,we know the plants ate the missing nutrients.

Now for the hard part,what nutrients make up the missing 0.4 ec & in what ratios ?

Lets say the plant is currently using high N but very little P & K,if you refill the rez with full strength nute water to where the ec is brought back up to 1.5 there is a good chance you have now raised the levels of P&K high enough to throw the mix ratio off balance,worse case scenario causing nute lock while still having a low ec.

Lest say your plants max ec is 1.5 without burn or lock out,if the plants are not feeding in even amounts when you top off the rez with full strength nutes its very likely one of the many nutrients will become so high it causes burn or lock out,it is never advisable to refill the rez with full strength nutes,at the same time its impossible to know how much of what specific nute to add,so we guess.

You should allways err on the side of caution,i mix my rez at full strength & then every other refill i add 25% strength nutes,one day plain water with the next day 25% strength nute water.

This issue is why its in the plants best interest to do a rez change once a week,its the only way to be sure your plants are eating the best they can.

You can push a rez change to 2 weeks safely by using an oversized rez but it needs to be nearly twice the volume to assure absolute optimum ppm levels,all in all changing out the rez once a week is the best route.

Cheers panhead for that info.

I do change the 2 small 20ltrs trays every week and even when I have my 100ltr header tank setup I'll still change both trays every week but I wanted to know who adds nutrients to there header tanks and who uses ph water.

Cheers

In my top off tank I only use RO water and some h2o2. I also use Dyna Grow so my ph is rock stable.

I think I'll fill the header tank with ph water to start with and see how diluted the nutrient becomes.
If it doesnt drop much in 7 days before I change the trays nutrient for fresh I'll roll with just ph water.
But if the nutrient strength drops too much I'll start off with 1/4 strength nutrients in the header tank and start from there.

Cheers all for your input on this.