According to the H&G feeding charts... Use Roots Excelurator until the 3rd or 4th week of flower (depending on flowering time). 6 & 8-weekers, until week 3; 10 to 12-weekers, until week 4. Basically until the transitional period is over and initial flowering has set.
Results? See below...
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(Top two pictures were of a DWC-grown Cindy 99 using Algen Extract, therefore the slightly brownish coloration. No disease, though. The bottom pic was of an Afghani #1 in DWC with no Algen. Those who've grown the #1 in soilless know how finicky rooting can be for this variety; that's the result in DWC, with two airstones. The roots snapped a stone in half by the time this pic was taken and were so enveloped around its pores that I had to let it go until harvest. Both pics were taken ar day 53 of flowering).
Better roots, better fruit.
Feeding schedules are available at
http://www.house-garden.us/feeding-schedules
"Argument is sugar, and the rest of us are flies"
-Richard Wald