The Tap Water Plus Conditioner Myth Debunked


One of the most detrimental mistakes you can make to a saltwater tank is to use tap water that has been treated with a water conditioner.

I could tell you why this mistake is costly to you and your tank. Instead, I put this myth to the test to see how “conditioned” tap water fares out in my test.

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Comments for this article (20)

  • Vasken says:

    Great post Mark! I’m looking into getting an RO/DI unit once I upgrade from my mini reef unit (15 gallon) so I can’t wait for your walkthrough. I’ve been using filtered water recently and sometimes i add conditioner oops. Thanks to you I’ll stop using it and cease to pump unnecessary TDS into my system. I noticed that you used tap water without running the water for a couple of minutes. I wonder if that will make a difference in the TDS reading.
    Thanks and keep up the great work!

  • Stephen says:

    I have been using tap water for over four months now. Never used a single drop of RODI water. I can say that I believe the reason my tank is successful is that I use an algae turf scrubber only… No skimmer, carbon or filter socks. This Method of filtration is how the oceans have been cleaning the water from day one. I am out to prove all people wrong on this, but only time will tell.

  • Stephen…turf scrubbers are a good bare-bones way to run a tank. I have a friend with one and he’s ditched his skimmer and hasn’t looked back.

    I think part of the reason they haven’t gained largely in popularity is that there aren’t any equipment makers who make them. Do you have website/blog for your tank build? I’d be interested to hear more.

  • Vasken…running your tap for a couple of mins will make the TDS go down some. When I first turn on my RODI system, I get about 140 TDS of the incoming water. After leaving it on for a while, the incoming TDS can drop down to 130.

  • Kam says:

    Keep up the good work..where is the next episode of picking up girls in LFS? LOL

  • Hmmm…I’ll have to think about that. Maybe in time for Valentine’s day

  • Damon says:

    Hi mark I live in hawaii and the way i get my salt water is througha pipe pumped straight from the ocean one of the major saltwater fish stores uses the same pump and my tank runs fine what do u think about that? Do u thinm that it is ok or do u think mixing my own would be better?

  • I think if you have unlimited free saltwater straight from the ocean – that’s awesome!

    The only thing I’d say is to have the stuff you need (salt/RODI water) on hand in case something goes wrong with the pump or the water coming into your tank suddenly goes bad.

  • Paul says:

    Hi Mark,

    Just wonder what happens if you stick your TDS meter in a glass of water comming straigth out of your tank.

    I guess it will be hight. But i also guess that you don’t have any algae.
    So if i am correct what the meter will read does not directly relate to how much algae will grow.

    I guess a better measure for how much algae you can expect from the water you put in would be to measure the phosphates and nitrate levels of the water you put into the tank.

    a RO/DI unit would be zero on those to test, but so might water out of the tap, depending on where you live. (or at least very low).

    Am i totaly off here?

    Best regards

  • luis says:

    Hi mark.. I’ve been using tap water with conditioner for year and a half now but its FOWLR.. my problem now is that I wanna put corals but they all die.. I put frogspawn several times but they always die.. my parameters r good.. I was thinkin about all the nutrients that tap water has is not good for my tank but its accumalative??? I bought my ro/Di unit a month ago but corals still dying.. what should I do?

  • Jessica says:

    Ok, so i have a problem. I just got a 75 gallon tank with the saltwater set up and set it up using tap water/conditoner and salt from the aquatic store. We already have sand/crused coral in there, filters and everything running and two live rocks and a base rock. What shoule I do? Drain it all and start over? how do i (or can i even) get the bad stuff out of my sand and filter…?

  • Jessica…no reason to tear it all down. Just start using RODI from now on.

    Your filter should be easily cleaned (although I’m not sure what type it is). Your clean up crew will take care of the sand, or the junk will dissolve into your water and through water changes you’ll remove it

  • Jessica says:

    Thanks, i have also read that it will be ok for a fish only tank, we were planning on doing a reef and corals, etc. Were about 4 days into it being set up, so it will be a while before we need a top off… (infact we need to take a little out as is) do u suggest maybe a partial water change? if we put the cleanning crew in, in about 2 weeks, and some fish in about 4, do you think the coral will die if we try putting it in around the time of the fish? or do we need to wait awhile so we can use more RODI

  • Luis Silva says:

    Hi there,

    I’ve got 170 ppm NaCi using Hm Digital Com-100 the tap water using it’s passing through aquamedic Easy Line 90 although I measure the water without filter and I’ve got 132 ppm NaCi same scale different values is this a normal situation ?

    What is the best way to lower the value for the tds ? Is there any product to apply on water ?

    Thanks in advanced.

  • Luis…are you saying you get the same value of NaCi for the tap water coming into your RODI and for the water leaving your RODI?

  • Andrei says:

    I’ve been using tap water & conditioner (Prime) for top-offs and for making saltwater for well over 2 years on my 20G nano reef and I haven’t had any problems what-so-ever. No algae, cyano or problems with corals (I mostly have softies, but also have an SPS and a LPS). Actually for the first year and a half I didn’t even have a protein skimmer, and since I’ve added one I haven’t noticed any change in corals or water parameters.
    But I’m lucky enough to live in an area with fairly good water (Toronto, Canada).
    If I was to set up a large reef aquarium with expensive fish and mostly SPS corals, then I would definitely invest in a RO/DI unit, but for a nano with small hardy fish and easy hardy corals I don’t think you actually need one.

  • Andrei says:

    PS: and of course the TDS reading after adding the conditioner showed a higher value than before, and it would have been a major surprise to see it go down unless you actually notice some of the suspended solid particles settling down and precipitating on the bottom of the glass (which doesn’t happen with any conditioner).

  • Luis Silva says:

    Yeah that’s correct if I measure in mS i get a value around 0,23 is this enough or a high value ?

  • Nancy says:

    Hello Mark,
    Thank you for this great article and experiment! We occasionally see would-be Reef Keepers give up in frustration after using tap water and experiencing chronic algae problems. For all our new Reef Keepers, we always recommend RODI water because, as with any other part of the delicate reef ecosystem we’re trying to replicate, if you start with the correct equipment and methods you will have greater chances of success. Don’t cheap out with garbage equipment and methods! Do it right the first time.

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