The Salty Side: Saltwater Aquariums for Beginners

Can I Use Tap Water in a Saltwater Aquarium?

As a new hobbyist, you may be wondering if it is okay to use tap water in a saltwater aquarium? After all, we use dechlorinated tap water right out of a faucet to top up fresh water tanks. Can’t be that bad, can it?

RODI water vs tap water

The issue with tap water is that it contains chlorine, nitrates, metals and other chemicals that can be extremely harmful to aquatic life.   Some areas have hard water and limescale is a huge issue for those living in those regions.  Flouride is another addition to tap water which varies depending on where you live.

You might hear of reefers saying that they use tap water and while you might get away with using it initially, in the not too distant future you will struggle to keep your tank free from algae.  This means regular algae blooms.  In addition, coral will not thrive and start to lose their color.  

Not only does this totally defeat the object of setting up a reef aquarium in the first place but will set you back a whole lot of time and money.  Not to mention disappointment.

Download our ebook on setting up a successful saltwater aquarium here.

The Salty Side

How to Purify Tap Water for Saltwater Aquariums

You will need to purify tap water using a Reverse Osmosis Deionisation (RODI) System to do this at home or you can buy RODI water from your local fish shop (LFS).  In the long run it will be cheaper to buy an RODI system to make your own purified water.  It will always be available when you need it and you won’t need to be wasting petrol running backwards and forwards to your LFS to buy more.

What is an RODI System?

Simply put, an RODI system is a water filtration system which can be used at home.  It removes chemicals such as chlorine, phosphates and copper.  If you’d like to more about RODI systems and how they work, see this article.

AQUATICLIFE Aquatic Life RO Buddie Plus DI Four Stage Reverse Osmosis Deionization Unit 50 GPD
  • ZERO TDS WATER- Produce up to 50 gallons of zero TDS water every 24 hours at an affordable price with the RO Buddie plus DI model, The RO Membrane will remove up to 98 percent of the TDS in water and the DI Mixed Bed Resin will remove the remaining TDS
  • SEDIMENT REMOVAL - The five-micron sediment filter cartridge traps particulate matter like dirt, silt, and rust with will affect the taste and appearance of the water
  • CARBON FILTRATION - The five-micron activated carbon reduces chlorine and conditions the water prior to the RO membrane

When it comes to topping up (or topping off) your aquarium, remember never to use salt water.  The reason is that when water evaporates from your tank, the salt stays behind.  This higher concentration of salt, along with newly added saltwater, is extremely harmful to fish, coral and invertebrates in your aquarium.  

Just starting out in the saltwater hobby? Get your copy of How To Set Up A Successful Saltwater Aquarium ebook today.

This ebook will help to guide you past the mistakes I have made over the years.

Can I use Distilled Water in my Saltwater Aquarium?

Can I use distilled water in my saltwater aquarium?

Whilst not ideal, distilled water can be used if you have run out of RODI water.  This is not to say that you should set up a tank using distilled water, but that it should only be used if you have no other option when topping up/topping off.  This should also be deionised distilled water.  

The major concern is that copper piping may have been used during the process of filtering the water.  As mentioned above, copper is not safe if you have invertebrates in your aquarium.    Another worry is that there is no telling how often filters have been changed and it is possible to get different grades of distilled water.  It may have been filtered but not to the extent that is necessary to keep a thriving marine aquarium.  


An RODI system is by far the best way of filtering water at home for your saltwater aquarium.  Not only is it cheaper in the long run but at least you know exactly what is going into your tank. 

Regarding distilled water, purification methods differ from one provider to another.  This means that distilled water isn’t necessarily the safest for your reef tank.  

At least if you’re using RODI water and there is a problem with your tank, you know it will be caused by something other than the water itself.  That said, it is a good idea to check your RODI water with a TDS meter to ensure that the filters in the unit are still working and do not need changing.  

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