Well, it might not work and it certainly has limitations, but so does every testing method or routine.
Hydroponics folks seem successful using it and it does simplify things a great deal.
Main thing is getting around the human part, most hate testing, water changes and other excuses to avoid work
You can go longer and tweak things a number of ways using EI, (at least 3 besides this TDS method using EI), but this might be fairly simple for folks also.
The point is to use the TDS to say when a water change is needed.
EI can be used in that manner and water changes applied using the TDS meter.
I have only recently found max levels for NO3, around 100+ ppm if you use KNO3.
So we have a pretty wide range before things get sour in terms of fish health.
PO4? No clue still. More than 5ppm.
Folks certainly can estimate for 2-3 weeks, then a TDS meter may help and extend the WC frequency out even further. And it's simple to use.
It's not that hard or difficult to estimate a tank's max uptake and dose accordingly. The TDS meter could be used to dose more conservatively to avoid as much build up and thus fewer water changes.
Edward also suggest using these for total build up for PPS for a water change. Nothing new here except applying it for WC as Frank suggested for EI dosing.
I suggest dry dosing of most ferts, he suggest his solution based ferts, EI can easily be used with solutions and his solution recommendations as well and add milliliters instead 1/4 teaspoons etc for greater accuracy and control if you so chose....but it's not needed to grow plant well. PPS suggest this, and adds pretty much the same deal, PO4, NO3, Traces and a GH, basically I suggest with EI 3 main things and GH if you do not do water change like on non CO2 plant tanks, or the GH is low or you go long peroids without water changes.
The end result is the same in terms of plant nutrient supply.
PMDD did the nutrients added to solution over a deacde ago for planted tanks. There was also some discussion on the APD, heck, I forget when about TDS usage for water changes.
Here's some old discussion about it from Wrigth from the bay area: http://fins.actwin.com/killietalk/mo.../msg00361.html http://fins.actwin.com/killietalk/mo.../msg00544.html
For killis and other fish, he uses a 2x factor before change is needed. http://fins.actwin.com/aquatic-plant.../msg00123.html http://fins.actwin.com/killietalk/mo.../msg00382.html
That's fairly simple.
Tom Barr www.BarrReport.com