RO/DI water is freshwater that has been filtered through a reverse osmosis system and measures with 0 total dissolved solids. 99.9% pure water without any measurable amount of elements, compounds, or metals that could potentially contaminant your tank or throw off the parameters of your saltwater. Owning an RO/DI system to filter your own tap water at home is pretty much a standard for modern reef tank owners because we ultimately want to provide the best environment we possibly can for our aquatics pets. 

Is unfiltered tap water really all that bad? Do we really need to filter it with an RO/DI system?

We know tap water can contain a wide variety of dissolved elements and compounds but could it be enough to really matter? It's time to find out just what kind of risk tap water poses for your reef tank and if dechlorinated tap water is a viable alternative to RO/DI water when you're in a pinch. 

The Experiment

To set up the experiment we took tap water samples from four different locals in Minnesota. We mixed up some saltwater with each sample using Tropic Marin Pro Reef Salt Mix along with a control batch using RO/DI water from here at BRS headquarters. We then tested 6 of the most important water parameters to see if there was any noticeable difference between the samples and control that would affect our tank inhabitants. 

  • Calcium
  • Alkalinity
  • Magnesium
  • Ammonia
  • Nitrate
  • Phosphate

The Results

Major Elements Results Nutrients Results

The control RO/DI water tested accurately to the advertised parameters for Pro Reef Salt. Based on nationwide tap water analysis, calcium, bicarbonate, and magnesium are all very common minerals that can be present in both municipal tap and groundwater sources so we expected to see some increases but certainly not to the extent that showed up in our tests.  

Major Elements (Calcium, Alkalinity, and Magnesium) - All four of the water samples spiked alkalinity with two of the four spiking the levels so high that it would most certainly pose a threat to your reef tank. Calcium was evident in one of the samples and magnesium remain unchanged throughout.  

Nutrients (Ammonia, Phosphate, and Nitrate)  - These are the results that were most eye-opening with deadly ammonia being present in two samples and phosphate increases across the board. Ammonia is typically caused by the use of chloramine which is a very common disinfectant for water treatment and is deadly to aquatic animals. Phosphate and nitrate are fundamental organic compounds that most hobbyists need to limit inside their aquarium to prevent nuisance algae and bacterial imbalances.

Can I Use Tap Water To Mix Saltwater For My Reef Tank?

Our water samples were pulled from a fairly small radius, each within 30 miles from the other, but we still measured some considerable differences. Truth is, tap water is going to vary wildly in terms of possible contaminants and dissolved minerals.  While there are probably a few tap water sources out there that are perfectly safe and won't cause any significant change to the saltwater parameters, it's difficult to know for certain without a sufficient water analysis report.

Based on the results, it is evident that tap water can pose a threat to your tank and should not be used to mix saltwater in 99% of cases. Not only do you run the risk of creating an imbalance of major elements, but there is also the constant risk of contaminants such as ammonia and phosphate poisoning your tank water.