There's no set rule, and you pegged it -- that it's misleading marketing that you're dealing with.

You can run your nutrients down before a reservoir change and they plants won't mind, they'll just taste better, first off. That way waste is minimized. Your plants don't have to be fed like geese getting pate made from their bloated livers.

People need to forget about changing frequency as a primary determinate, and look at how much their plants are eating. Without knowing how many liters of solution there is per plant in the specific set up, the plant growth rate, and the solution strength, to talk about a changing frequency is kind of useless, unless you are a salesman selling nutrients.

All you need to do is manage your EC range and you decide when your reservoir is dirty enough or if your plants seem not up to par. Mine are currenty outside iunder Florida folliage and spores and crap falls in all the time so I change every 6 weeks.

I always run it down a little at the end. How much nutrient you add depends on how much your plants actually grow. How often you change is more related to how dirty things are getting. If your plants grow nicely, there is no need to do what amounts to waste more unused nutrients. But you do need to keep the EC range ok, not to maintain full strength of nutrients, but rather to meet the plants need to generate root pressure so the plant can effortlessly take up solution instead of fighting an unfavorable concentration gradient.

Hey,
All The Hydro shops around here must be in cahoots together. They all suggest changing the res water every week LOL.
I feel the same way as you do, and think it's a waist to throw out the res nutrient so often. I'm thinking it's marketing too.

I however have not made it past a week, before having to change out my nutrient solution due to a big learning curve :-)
My pH keeps swinging up and I need to add more and more acid to keep my pH below 8. At the end of the week my res is just full of nasty sludge at the bottom, is off color and smelly.

I have just switch brands of nutrient and it so far is out performing my old nut. Once I get this figured out then my hope is to run three to four weeks on the same res before having to change it out.

you probably have hard water,Astaroth.
learn to adjust and stabilize your pH with plain water. Once you've got it all stabilized, set it at neutral (pH7) then ad your nutrients.
No need to waste nutrients trying to figure what's up with your water.

@ Grizzman
Not sure what the tap waters hardness is,would need to get a report from the township. The water is pH 7.5 out of the tap.

looks like I may have the pH swing problem solved.
I switched from DNF (Dutch Nutrient Formula) to Dutch Pro.
The Dutch Pro needs no pH adjustment and is staying locked
in at pH 6.0, where I want it.

Glen

BUSTED..... I used to change my nutrients EVERY WEEK. After keeping a log and always testing my PH and PPM I realalized it was not necessary AT ALL. The few things I have learned, I can top off my drum with some rain water if i need to keep the level up and the PPM just goes down a few 100 points. Not enough to cause a problem with the plants. However since i have turned down the FLOW into the pipes i use / LOSE less from over flowing our my cut out. I was noticing when the flow is past a certain amount the water does not have time to get past some of the root balls and will drip out the pipes. Like PupillaCharites said just keep and eye on the strength. At the end of my last lettuce crop i was just feeding them rain water and they were fine. Now Spring is here and i hope to take pictures and post some new videos of my system,

awesome advice everyone. Thanks! Apparently the hydro store ain't out there for the common people like us who can grow just fine without the BS. Thanks for the support

Its best to keep an eye on the plants rather than the nute strength, they`ll let you know if somethings amiss long before a meter. I only change my res when its empty :)

Hex2006 you are right on with your comments about nutrients, I have just returned from overseas being absent for about two weeks, my plants are doing extremely well and have grown by about 2/3rds .
I checked both the ph and the Ec and they are spot on so I will let the plants grow happily on without interfering with the ph and ec levels , as you mentioned if the plants are not showing stress leave them alone.