Nutrient Dosing

First Hydro grow. Made a simple rdwc system out of 17gal totes. Only one grow site and one rez. I have a question around water maintenance.
Fresh water/change water I understand the nutrient dosing. However, I am reading that I should be targeting water changes every couple weeks. If that is the case my question is around dosing those in between feeds. I am targeting a specific ppm but I don’t understand how I would properly mix my nutrients (GH Flora trio) to get to the correct ppm/mixture. ie if my water is at say 400ppm after week one and I am targeting 900ppm in week two. I can’t add the recommended nutrient volumes laid out in the Flora Series feed chart as that would likely yield too high of a ppm. Let alone knowing what the current nutrient breakdown is of the initial 400ppm starting point. So I am a bit stuck on the feeding unless you tell me to change rez water each week with each feed. Then I would say that seems like a ridiculous amount of water usage as my system holds 25gL. I have not found a good resource to help with the feeding cycles.

Any advise is appreciated.

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Hey @BeezNeez
When adding to your 15gal 400ppm system and you want to end with 25gal 900ppm.

Calculate what it would take to bring your system up to 900 from 400.
You’ll need to know your full capacity 25gal currently at 400ppm at 15gal. So you’ll be adding 10 gallons that will bring the total system of 25gal to 900ppm.
25gal x 900ppm = 22,500
So you’ll need 7500/10gal to bring your current system to 900ppm.
Plus you’ll need 9000/10gal for the added 10gal to total 900ppm.

Here’s the math. :abacus:
Currently in system 15gal @ 400 = 6000
You’ll add 10gal @ 1650 = 9000 + 7500 additional to up you current 15 gal from 400ppm to 900ppm

Apologies in advance my gummy kicked in.

Where you are losing me is with reference to 15gal. I assume goal is to maintain somewhat regular water levels which would need water top offs? Or is this not accurate?.

I believe he was just giving a for instance. Meaning if your system held 25 gal of water. And only had approx 15 currently in it. With a ppm of 400. He was showing you the ppm of the 10 gal you would have to add to make it achieve your goal ppm with 25 gal.

Got it now, that makes sense. Thank you both. So for my info will I loose that much water? Or should I been topping it regularly to keep near the 25gal?

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I’m not a hydro guy yet. I can tag @Graysin whom I believe is one. And @peachfuzz
They can let you know how low you can run a system before needing a top off .

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Thanks mate. Appreciate the tags.

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Thanks for the tag.

I’ll explain how I handle top-offs in my reservoir and you can evaluate how to handle yours if you’re similarly situated.

I run my RDWC with one 27 gallon tote (where I do water level, temp, ppm and ph checks, and have air stones for oxygenation). I then have 3 5 gallon buckets which actually house my plants, they are interconnected in a square with 2” PVC. The pvc runs from my res to Bucket 1, then to Bucket 2, and finally Bucket 3, which contains my water pump that feeds through the water chiller and back into the main res.

I initially will fill my reservoir up to just above the top of the PVC - this ensures the water is going to fill each of the buckets along the route before returning to the final bucket and water pump. I will re-fill my reservoir when the water level is depleted below the bottom of my PVC pipe - so a little bit over 2” of water has left the system. For me, that translates into somewhere around 7 gallons.

When topping off, that means I will add Week X nutrients at a rate of X per gal at 7 gallons. This affords me some wiggle room, because the nutrients are full strength for what I’m adding into the system, but possibly “low” for my target PPMs. But it does mean I will have replenished approximately what the plants have consumed. After adding nutrients and “top off” water to my reservoir, I let it circulate for a good 10-15 minutes, then come back to check ph and PPMs. If it’s not quite where I want it, I’ll add a small dash of the nutrients I need (I use Jacks 3-2-1, so I’ll just add in a little more of the full strength blend). If the PPMs are too high, simply add more RO or distilled water. As long as you aren’t in danger of covering the bottoms of the net pots, it’s okay to have a little extra water.

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Thanks. That is helpful. How often are you topping off? Also sounds like before I start I need to make some water markings on the Rez at 1-2 gallon increments so I know who much water loss I have. Paint pen okay?

How low do you let the water get on the root line? I am assuming in the army stages when roots are short this requires more frequent topping?


Depends on the stage of growth. I top off once every 2 weeks in veg, around once every 4-5 days in the transition, and once a week(ish) in flower. They get hungrier the bigger they get.

I think so as long as it’s oil based so it isn’t water soluble.

Til my roots are totally exposed. I’m kind of a savage to my plants. I am more worried about overfilling than underfilling.

That hasn’t really been my experience. The roots will grow down to hit the water. I let them have a 3” gap between the bottom of the net pot and the water line, I just ensure the bursts of water droplets still reach the roots.

IMHO, just to keep it simple, if you have two figures,

Current ppm’s = 400
Targeted goal 900 ppm’s

Let’s use 100 as a targeted goal for an example.

Current ppm’s = 50
Targeted ppm’s =100

50/100 = .5
100/100 = 1
.5 + 1 = 1.5%

So, you would make up 10 gal at 1.5% times whatever is required.

So, if GH calls for .5 tsp/gal of “Grow”,
you would multiply .5 tsp per gal times 1.5% = 3/4 tsp per gal of grow instead of .5 and the same for the rest of the nutes.
After all the stuff is added, that will bring your total volume up to 100

Using your current situation:
Current ppm’s 400
Targeted ppm’s 900

400/900 = .44
900/900 = 1
.44 + 1 = 1.44 %

So in your case, you would make up 10 gal at 1.44% stronger than normal to bring the total volume to 900.

That’s the way I do mine. I have two 20 gal totes (2 grow sites) to grow with, and an 18 gal trash can for a res. It holds 22 gal total.

There shouldn’t be any reason for your nutes to drop that low unless you don’t have enough volume of water which you do.

In a perfect world, your water level will drop and your PH and ppm’s will remain the same. If it doesn’t, then there are other reason why it dropped, which you’ll cross that bridge once you get there, if it ever comes to that.

As far as water level goes, I start out at 2” and it won’t drink much water for the first 4 weeks or so. After roots hit the water, my water level will sometimes drop 4-6 below the netpot. You just don’t want your water level right under the netpot. They will drown.

Feel free to tag folks if you have any questions. Good luck! :+1:t2::v::sunglasses:


@Graysin @HappyHydroGrower
Thanks boys, I think I got enough to be dangerous. Just started my seed last night so I will update you once I get her in the net pot.



I have a ~35 gal RDWC system with four plants. I use auto-top-off to keep the water level steady. I check PPM daily, and unless something weird is going on, I add nutes in the correct order and correct proportions around once/week. Typically, this means I’m adding nutes for 5 gal (e.g., if it’s 2ml per gal for GH Micro and 4ml/gal for GH Grow, then I’d add 10ml Micro and 20ml Grow).

I have a very robust mixing/circulation setup – seems kind of important for the Silicon nutes (which have a high pH), but that may be overkill (and indeed, you might not be using Silicon).

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Alright seed has popped, I will put in rock wool tomorrow. I’m the mean time I have been running my system all week just to get a handle on where the Rez temps settle and experimenting with a couple cooling methods. Tonight I am PHing my water down to lower 6s just to run there for a couple days. I did measure the ppm of the water in the Rez and it is sitting at around 480. Seems high and had a question on how I mix my first nutrient mix. With a starting ppm is that going to cause issues?

Just trying to stay a step a head as long as I can here.

@Graysin @HappyHydroGrower

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I need to calibrate my ppm meter. That reading seem shady as hell

Is that water with your nutes in it? Or is that tap water? Or tap water with nutes?

I’d be wholly comfortable feeding a seedling anywhere between 200-600ppms.

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@Graysin That’s just city water out of the hose. I did use a Boogie Blue + filter on it. I am in AZ so I know our water is hard and full of shit. Still this seems high and need/want to calibrate the ppm meter

Yeah, I’m in central NM. hard water is standard

Check your TDS meter again with just plain tap water. Don’t pH or add anything when you test it. That will be your baseline PPMs.

My tap water baseline is about 250ppms.
So when I feed with tap water, I subtract 250ppms from my total PPMs after amending the water. I.e. my target for feeding my girls is about 1500 PPMs- so when I feed, I actually look for a readout between 1700-1750 PPMs (to account for the extra 250ish that my tap water provides).

Edit to ask: how old/new is the filter? Try the tds meter with filtered water and again with non-filtered water. I’m not sure why, but I’m suspicious of your filter.

Brand new filter but good call on testing unfiltered tap water vs filtered. I also have a second filter that is a few years old but technically has a ton of gallon capacity left. That’s should give me a few good reading to compare and get a baseline.

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just ran some water tests with my ppm.

Distilled water: 0002ppm
Tap Water: 0395ppm
Old Filter: 0395ppm
New Filter: 0500ppm

What’s the deal with this? New filter cup of eater was completely new from Rez, same ppm. How did this “filter” add ppm to the tap water?? I guess it’s only job is to remove Chlorine and Chlorimine compounds. Perhaps it’s adding something to break down the CH compounds?