What is best for topping the res off during flowering phase? And how often should I replace the the res during flowering phase?
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I always added PH balanced and filtered water every day until the res reached the level I marked in the tub. After a few minutes I would then double check the res’ PH and EC and make adjustments as needed; usually just the PH.
When doing RDWC or DWC I found that the best practice is to totally replace the solution in your res 1x a week. I did a test and found that you can push 2 weeks if you top off your nutes after week 1 but going past 2 weeks caused problems.
No matter what I’d get a lockout of some sort because the plants take up what they want/need and unless you got lab testing eq and trace elements in their parts good luck matching what’s needed 
Initially I was annoyed at the thought of such waste, but i have a lot of houseplants so I use the old res solution to “water” the plants in the house or outside plants; even in winter they’ll get the nutes after the thaw 
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Thank you!
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I’m the rusty side of the DWC coin. Pidge is a better grower than I am, but I’m high, too. Make your choices.
I change my rez bucket at flower, or if I have problems. Problems are usually uncontrollably falling pH or root fly larve
I add filtered water during the week, and pH the bucket. On the weekends I add nutes if needed. Sometimes I’ll feed mid week when they are early/mid flower.
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On the other side of the coin, I just monitor tds and adjust topoff by feel and stage and rarely get a deficiency or issue. I almost never do water changes and have run successive grows in the same water/bucket at times. And I only use two part plus random mg on occasion.
I’m not necessarily suggesting it, though if you’re experienced and haven’t tried it I would suggest giving it a shot. I didn’t do a water change at some point a few years back because I wanted to see what would happen and it ended up being a phenomenal grow with huge yields. Haven’t changed water in hydro, except between grows, since.
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I appreciate that it works for you cause it means that if you are in the right situation with the right variables you can accomplish great grows without having to faff around so much! 
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I was pretty shocked, everything I’d heard said you shouldn’t do it, but it just didn’t make sense to me that cannabis would need water changes like so many claim. I figured I’ve learned how to grow coral without water changes, cannabis should be easier…
I run nitrogen heavy to start and through veg and early flower, then topoffs are primarily p and k with minor n
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What hydro system do you run?
I bet if I ran larger volume rez I could get away with more. I’ve taken a couple jabs at RDWC but it hasn’t stuck yet.
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What I’ve used the most is about 45 gallons total, 8 sites with a 30 gallon res that’s filled 2/3 the way.
I’ve also done it with single buckets, but that’s easy IMO since there’s so much replenishment.
This is the way for repeatable predictable results. Well written @Pigeonman Cheers !!! I know there are other ways and guys have great success meeting ratios with the same batch for weeks. But I like predictably and consistency of weekly changes. It’s more mathematical where as the same batch approach is more of an artist approach some guys do well with it others struggle with it. The real benefit in the single batch approach is no stress on the plant as they are not fond of any changes in PH or temp or RH nutrition ect. The game is keeping the stress minimum whatever your approach. I like to say I mastered the art of the switch and single batch guys master the art of the mix. Cheers Everyone!!!
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