Water Treatment

I recently discovered that my source of RO water is the most likely factor in my past few grows being progressively more unsuccessful than the last. I seem to be getting much better results after switching back to distilled. The RO water seems fine if I take the time to sterilize it but it’d be easier just to use distilled water if that’s going to be required.

Do you treat your water? What do you use? Why?

I’ve read that chlorine/chloramine can be used in small doses to maintain a sterile res, and that it’s more lasting than h2o2 treatments.

Lately I have been pre-treating water with h2o2 and after 48 hrs adding hydroguard then adding it to the res.

I don’t want the increased risk of spill that comes with an external water chiller, so finding a solution that remains contained within the grobo (in it’s “factory” state) is my goal.

My biggest question about this is really has anyone else gone down this avenue and what have the results been? Did you notice any reaction from hoses/rubber/etc after prolonged use or anything that would be useful to know?

A side question for those using hydroguard – have you heard about the “3 week rule”, do you follow it? That might be why it didn’t work so well for me the first time; I’m about to do a split test to see if it makes any difference.



Where are you getting your RO water from? I don’t think they are all the same. Buying it from grocery stores from their machine has no chemicals like getting it from Gordons or Besco water company where they may add things to the water. I always add a gallon of distilled water to my RO water and check it with my PH Pen.

I use Distilled gallon jugs from the grocery store. It seems that less nutrients are needed / dispensed and the pH is not in flux.


I’ve been getting RO from the machine at walmart using 2x 18.9L refillable jugs:

  • It’s been coming in around 8-12 PPM (17-25 µs/cm) with PH of 6.3-6.6, it’s always slightly different.
  • The same machine was used (weeks apart), used about 80L from this source so far (4x 18L bottles total across 3 trips).
  • Here’s the product itself: https://www.walmart.com/ip/Primo-Self-Service-Refill-Water/151610983 (it’s about $2.39 for 18.9L at my local store).
  • I haven’t been able to find anywhere else to actually refill the jugs, and no other stores that claim it’s reverse osmosis water (just jugs of distilled).

I’ve been getting distilled water from the same walmart, and from grocery stores in 4L bottles:

  • It’s been consistently coming in at 0 PPM (0 µs/cm) with a PH of 5.7
  • The same PH/PPM was observed across samples from 3 different brands from different stores, used approx 24-30x 4L bottles.

The probes have been checked and calibrated and are at a very low error threshold (within or less than 0.1%), so I do trust the data. Each reading was checked and re-checked with attention to avoiding cross-contamination; when this did occur it was noticed. :man_scientist:

Took a quick look under the microscope, somewhere around 60x magnification:

  • Distilled:

    • In other parts of the sample there were many of the “rainbow drops” seen like that one just above the center in this pic.
      • That’s interesting to note because none were observed in the other water.
    • The edges (or “lines”) also appear to be allowing a fuller spectrum of color through, some appear blue or purple or even red.
  • RO:

    • There is just so much blue here!
    • One thing that caught my eye was how at the edge in some areas there appeared to be some sort of clumps, one is visible at the top of a drop just below the center in this pic.

Ignore the ripple-looking circle as well as the freckle-looking specs that are slightly out of focus in those pics, that’s not part of the water sample. This isn’t the most expensive unit around but it works for the kids’ needs. In this case the intent was just to compare the two and get a closer look at what I’ve been feeding my plants.

Any ideas what that might be in the RO water? If it were chlorine I’d expect it to have done a better job at helping to stave off root rot.

This discovery is enough to cause me to stop using it, especially considering the fact that the distilled water from multiple random stores has been consistent in terms of PH/EC. On top of that, my grow (when using distilled water) was fairly successful even though I was having issues with nutes dispensing due to my unit overheating and shutting down at the most inopportune of times. Things really started to go south when this RO water was introduced; so a word of caution when recommending people switch their water!!


Not a clue what it could be, residuals from the system but, I do know reverse osmosis water filters will reduce a pretty wide spectrum of contaminants such as dissolved salts, Lead, Mercury, Calcium, Iron, Asbestos and Cysts, it will not remove some pesticides, solvents and volatile organic chemicals (VOCs) including: Ions and metals such as Chlorine and Radon.
P.s. I’m not sure where you live but if you put the words primo water in your search engine they will show you all the places/stores you can refill your bottles with RO water. Or like some allgrowers would suggest, get a home RO system they are cheaper than refills and saves a lot of gas and time.

If you plan to be in the game for any real length of time, you’ be doing yourself a major favor by simply buying a home countertop 5 stage ro system. It pays for itself very quickly when you calculate how much money you spen each trip on water and gas. It’s just one of those “work smarter not harder” type of things :+1::v::joy:


I think that will be my x-mas gift to myself…

1 Like

No I have not heard about the “3 week rule” what is it?

I’ve read that when using hydroguard (or similar product containing “Bacillus Amyloliquefaciens”) it’s best to not change the water for the first 3 weeks to allow the colony to establish itself. The idea is that by dumping the res too early it just replaces the “mature” colony with a new immature colony essentially starting over again.

I personally haven’t been using hydroguard much past the 1st month with my last 2 grows with that in mind, as a bit of water is left in the res between drain/fills – hopefully enough to keep enough of them around. I also haven’t had a recurrence of the issues that originally caused me to start using it, thankfully. :crossed_fingers:

Also note that waiting 3 weeks for that to happen isn’t really possible to do “out of the box” – the grobo requires a drain/fill to occur in order to feed from the bottles. Waiting 3 weeks to do a drain/fill would leave the plant without food for 2 weeks so manual feeding would need to occur during that period.


for the info I’ll stop using hydroguard using the 3 weeks guidlines too

1 Like

There isn’t any harm in using any more than is required, but from what I read there’s not any benefit either. A “mature colony” is the goal in order to fight off potentially harmful bacteria/pathogens.

The key words there are “than is required”. It is different from case to case and is also highly dependent on the environment/conditions the plant is growing in.

In my case, the room my grobo is in was subject to warm temperatures too often and the reservoir was sitting around or just above 73°F for some parts of the day on hotter days. Addressing the A/C issue made a much bigger dent in the problem.

Monitoring the reservoir temperature is the easiest way to determine if it’s even necessary to be adding – if it never hits 70°F+ then it could just be used lightly “just in case”.

  • If it stays on the warm side then you might want to add a bit more to maintain the offensive.
  • If there is a bad smell coming from the res then it might not have been enough;
    • and in that case I’ve found that the res needs to be fully emptied and cleaned again to increase the odds of survival/recovery.

The one thing to really point out is that over time you’ll figure out what works and what doesn’t. Much of this is just figuring out what works in your case. Be careful about taking advice! A bit of experimentation and note-keeping goes a long way, especially as the weeks pass – it’s easy to forget or miss something given the long duration of a grow.


I have read the same thing but, have never had a problem since using hydroguard (every week) like I did before I used it, and my water temps stay around 73.2 - 73.9


Yeah I’ve not noticed a difference, I’ve been using it every week for my current grow. Mind you I might not have needed it, being that it’s the winter and my res temp isn’t what it will be in the summer. Now I’m using it moreso as a precaution. Rather be on top of it before anything can fester lol