Salty Q&A: Is RO/DI Water All It’s Cracked Up to Be?

RO/DI treatment is a hedge against tap water which typically has impurities that can cause issues in our aquariums

RO/DI treatment is a hedge against tap water which typically has impurities that can cause issues in our aquariums


I’ve been keeping saltwater aquariums for five years, and in all that time I’ve never used RO/DI water for top offs or mixing up clean salt water. I just use tap water treated with a dechlorinator / dechloraminator, and I’ve never had any problems. Could it be that people are exaggerating the benefits of using RO/DI?” – Submitted by Lee H.


Though you may have gotten by using only a dechlorinator/dechloraminator to treat your water thus far, you may not always be so lucky. I’m a strong proponent of using RO/DI-treated (or otherwise purified) source water in any marine aquarium system for several reasons. Here are just a few:

Water treatment protocols are for people

Municipalities aren’t (nor should they be) worried about providing water that’s safe for sustaining sensitive marine organisms. Their only concern is ensuring that the water is safe for people to drink, bathe in, etc. So, you can’t assume that whatever chemicals/additives are present (beyond the chlorine/choramine you’re treating for) are safe for fish and corals. What’s more, municipalities often change their treatment protocols with little or no public notice.

Bad things can happen to otherwise good tap water

Whether as a result of human tinkering or natural influences, previously safe, potable tap water can become contaminated. The current situation in Flint, Michigan is an obvious case of the former. An example of the latter occurred much closer to home—right here in Toledo, Ohio.

Located adjacent to Lake Erie, we’ve long enjoyed high-quality tap water in abundance. But in recent years, massive summertime blooms of Microcystis algae in the western basin of the lake have threatened the safety of our water supply. In fact, two summers ago, the problem reached crisis proportions when dangerous levels of microcystin (the toxin produced by Microcystis) temporarily rendered our water unsuitable for drinking.

In any such circumstance—or even less dire ones—I’d rather not take a chance that the tap water will be safe enough for marine life after merely treating it with dechlorinator/dechloraminator.

Less guesswork when problems arise

Whenever you experience a problem with your aquarium, such as a major algae outbreak, you have to go into Sherlock Holmes mode to figure out the cause. If you’re using RO/DI-purified tap water for top offs and water changes, you can safely rule out that the algae bloom is being fueled by nitrate, phosphate, or other nutrients present in your source water (assuming the membrane and various cartridges haven’t exceeded their functional lifespan, of course). In other words, with RO/DI, you’ve got one less thing to worry about!


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About Jeff Kurtz

Jeff Kurtz is the Co-founder/Editor of Saltwater Smarts, former Senior Consulting Editor for Tropical Fish Hobbyist Magazine, and the aquarist formerly known as “The Salt Creep.” He has been an aquarium hobbyist for over 30 years and is an avid scuba diver.


  1. I had a horrible experience which made me swear off tap water forever, even for freshwater fish.

    Long story short, I did my water change on my 50g freshwater planted tank as usual, about 30-50% with Prime dechlorinator – same as always. What happened next was not something I’d ever expected. By the time I finished refilling, I realized that my fish were dying within seconds of each other – just bam, bam, bam. I grabbed a net and started throwing fish into a quarantine I’d set up which had not had its water changed yet. I managed to save some, but I lost over 3/4 of my fish that terrible day…now I tell people to use rodi with a product such as Seachem’s Replenish. I don’t want anyone to go through what I did. Come to find out several of the other aquarists in my city had suffered the same that day.

    • Jeff Kurtz says

      So sorry to hear about your losses, Lisa, but thank you for sharing this. No doubt others reading your account will benefit from the wisdom of your experience.

  2. I had the same problem as Lisa. I used tap water in New York for 5 years with no problems. I used to filter it through a PVC pipe filled with carbon. One day I did a water change and my fish started dying almost instantly. I lost an 18 year old cusk eel and a ten year old mandarin along with most of my other fish. I found out my town added zinc orthophosphate to the wells right near my home. Then I got a RO/DI.

    • Lisa Foster says

      I remember hearing about that. Sorry for your losses, too!
      I wish they were mandated to tell us all BEFORE a change is made.
      Not that I’d want to use Hemet water anyway. Tested it out at about 700TDS at one point…give or take 100.

  3. What are your thoughts on using a 10, 5, 1 and .1 micron filter system without employing the use of an RO membrane in an effort to reduce the amount of water wasted?

  4. Jeff great article! I am all too aware of the benefit of a quality RO/DI system and I use the word quality literally because like any other product all RO/DI products are not created equal. Unfortunately many users hear about others using tap water and they do not realize that the source of water makes all the difference. I actually have very good source water but because of the reasons you mention I run a Six Stage SpectraPure RO/DI with 99% membrane. Not only do I feel it is my responsibility to do everything I can to protect and provide for the organisms in my system but to any one even considering setting up an system better to be safe than sorry.

    • Lisa Foster says

      For anyone wondering, I can second that Spectrapure is a terrific brand. I was using BRS filters on my unit for a long time, but when I was advised to switch to Spectrapure I went ahead and tried it. The filters, especially the DI, lasted tons longer. I now have one of their units (I lost all my fish and equipment when my husband dumped me off) and will continue using their products.

      Their customer assistance was very nice when I needed it, as well. They accidentally send me a non-color-indicating DI filter when it was a CI one included in the pack, so they sent me a new Color-indicating DI filter – no returning the old one, not even a shipment fee, and the people I spoke to were very pleasant and helpful.

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