Almost every newcomer to the hobby asks the same question…Can I not just use tap water in my tank?...The answer to this is a resounding unequivocal NO! This article will hopefully tell you why using tap water can lead to disaster in a marine tank.

Natural sea water (NSW) contains hundreds of trace elements that used by the various species found on the reef from fish to corals and invertebrates. Due to the enormity of the seas around the world, these elements are extremely stable and the creatures that live in them are unused to substantial changes. It is therefore extremely important that the water in your little slice of the ocean is in top condition.

It is quite natural to think that tap water is safe for human consumption so it must be safe to use in a fish tank. However, there are many chemicals added to water that are completely unsuitable for a reef tank. One of the major problems from using tap water comes from fertilizers. These are used by farmers to grow crops and as such contain huge amounts of phosphate and nitrate. When it rains the fertilizers are washed into the water supply. The water company does not remove them because they are not dangerous for humans however, in a reef tank high levels of phosphates and nitrates can lead to a major problem with algae which is unlikely to go away since every time you perform a water change you are literally feeding the algae.

If this wasn’t enough to convince you, there is also a more life-threatening (for your livestock) consequence. The majority of water pipes are made from metals, especially copper. Water travelling through these pipes picks up some of this copper. If it is introduced into your tank, even at low levels, it can be fatal to all invertebrates within your system.

Tank started with tap water

So what is the solution? The best thing you can do when setting up a tank is use water which has been through a reverse osmosis (RO) unit. This removes all impurities from the water and gives you a blank canvas with which you can add salt and any other trace elements you see fit. On average a RO unit will remove 95-97% of all total dissolved solids (TDS) from tap water. This can be increased to around 100% by adding a de-ionisation (DI) stage to the unit.

The cost of a RO unit varies on the amound of water you require from it but for a typical 40 gallon tank, a RO unit can be picked up for around $100. A RO unit is quite wasteful with water with around a 3:1 waste to actual RO output therefore some people also worry about the cost of additional water use. However, when actually calculated the additional cost is very small. If you want to do a 10% water change on a 40 gallon tank, you will need around 16 gallons of untreated tap water. This costs around 16cents per week. If the cost of buying a unit outright is too prohibitive, you can buy RO water from your LFS for around $1-2 a gallon however, you will need to test it as some LFSs have been known to sell less than high quality water.​

As a closing point, it is important to add that the cost of owning a RO unit is much cheaper than the cost of extra equipment and effort required to battle the inevitable algae problems you will suffer. It cannot be stressed enough how important using RO water is. If you don’t have one, buy an RO unit now!

no algae.JPG
A tank with no algae
Last edited by a moderator: Sep 27, 2013
mblack, Jul 2, 2009