Could be that your fert is formulated with tap water in mind. R/O water is anywhere from 50 - 100 ppm low in TDS compared to tap and i can easily see that as the difference in the final TDS. But my main question is: are you sure the pro-tekt can be used to set the ph of the stock solution and come out right when diluted in the working solution? Are there not products actually made to be used this way that will give you the dilution ratios without you having to apply commonsense math which may or may not work out the way you hope because of interactions of other variables?

H

I never even try to be that precise. However, under a strict regimen to maximize growth rate in a commercial setting I suppose that is the best way to go. I assume you took solution temperature into account.

The way I would approach this is first, to weigh rather than measure. Second I would use the empirical method of mixing and measuring until I got the reading I wanted. If you keep good notes, about the third time you did it the concentration would be very close on the first measurement.

Thanks Howard and Richard, I guess I'm trying too many things at one time. I will try again keeping the fertilizer and Pro-Tekt separate and see if that works better.

Thanks again

Joe