RO vs. Tap Water

I’m curious what the detriments of using Tap Water (I live in a major city). How important is using filtered water to the development of the plant?



It Can Be Hard To Say Exactly. Just Because Each City Treats Their Water Differently.
The Chemicals Used To “Clean” Or “Process” The Water, Can Be Harsh. The Sensors In GROBO Are Sensitive And Can Be Temperamental When Exposed To Said “Chemicals”.
I PERSONALLY Use Distilled Water And Have Not Had An Issue… Knock On Wood :facepunch: 🪵


Distilled water also.


So…what about a tent grow? I don’t have a grobo, I use a tent. The only sensors I use are TDS that I check at feeding and PH that I check every day.


If you take any Tap water and use a TDS meter and could test it for chemicals you would see it varies and has a lot of variables. Distilled water the plant is purely getting the nutrients and nothing else giving you a controlled environment as well as the best thriving environment for your plants. Not so much a PH thing.


My tap water shows 125 PPM. I guess I could do a grow with tap and one with RO, same strain, same nutes, and see what he effect is…


If you are going to use city water that has any chlorine in it you’ll want to put it in a bucket and let it sit for at least 24 hours to allow the chlorine to evaporate. @ToddYYC, @Mpower11, @chris_barfield may (probably) have a better explanation about this.


I do not. I do not use tap water. The calcium buildup aftet just 1 grow would ruin the unit.


I’ve done it both ways pros and cons to both

Biggest pro with town water is it’s cheap and easy.

Cons with town

  1. chlorine will kill off all your beneficial bacteria‘s. When I used town water like @VermontGrobo said I filled food grade buckets with water with the lids off in a dark room to let the chlorine gas off for 48 hours before I used it.

  2. I agree with @ToddYYC calcium in the water it’s hard on the equipment (humidifiers and probes

  3. Some people town water is to high in ppm for starting point

Mind you if you town water is in acceptable ppm range you will not have any issues with your grows.

I run R/O now with everything because I want my plants free of contaminated water.

What is nice when you use R/O water your starting off with 0 or 1 ppm. When your making your nutes you have a clean start to properly Dial in your Ec.

Cons to R/O really just comes down to cost you have to replace your filters after Xx amount of time they are not cheap

You have to setup a holding tank to storage your R/0

One word of caution with R/O water in a tank with a float system is you have to watch out for is what is called “TDS Creep”

TDS Creep is the high TDS water that an RO unit makes when: A) the initial water made when starting up the RO water has high TDS as it is making water from the stale water that has been sitting inside the unit, or B) the water made by an RO unit that is under low pressure, such as when the RO unit is being used as a top off system where the unit makes water to match the evaporation rate.

I had my Ro system pumped directly into my humidifier because it started and stopped so much I TDS creeped my humidifier and my whole tent. It wasn’t pretty

Here is my R/O system

This is my holding tank

Here is my humidifier it’s a warrior!! Never have to fill it up plumbed into R/O water supply


@Mpower11Thank you for that info. That makes a lot of sense. One other question, you mentioned the water being in an acceptable range it would probably work. Would you consider 150 PPM acceptable range? Or is it not that straight forward?


Anything under 250 ppm is acceptable but remember, starting with TDS rich water is like painting a picture on a smeared canvas, the cleaner the canvas the better for optimizing mineral availability.

Hope this helps


It’s like the difference between drinking 18 year old Scotch and 6 week old whiskey that comes in a plastic bottle.