The topic of water changes for a saltwater aquarium is still a topic that generates a great deal of debate. Part of this is due no doubt to wishful thinking, part due to claims by some hobbyists that they have never done a water change and part due to there being no hard and fast rule as to what is ideal. I still remember vividly the claim of an early trickle filter manufacturer that part of the allure of their filter was that it "Eliminates the need for water changes forever". Unfortunately, experience has taught us that these claims were not only untrue, but were deleterious to our animals as well.

Better filtration methods have allowed for most tanks to no longer need to be completely dismantled in order to be cleaned. However, the daunting task of doing water changes still remains. I would love to tell you that it is no longer necessary to do water changes and I'm sure that many of you have tried or know of someone who has not done a water change for a very long time. Some of these individuals may even have very successful tanks employing this strategy. However, for most of us that overstocks and may overfeed our tanks from time to time this strategy will not work in the long run.

The reason for this is simple: despite all of the breakthroughs in filtration, both chemical and mechanical, no method that I have seen yet removes the harmful substances that accumulate in a closed system. A water change is not just a way to replenish elements that are consumed in a reef tank; this could more easily be done by adding trace elements from a bottle. Rather, a water change is the only way to remove harmful compounds that accumulate over time which filtering does not remove. Even though not all waste products are removed by doing a water change, any that are still in the water become more diluted. This is why when doing a water change it is best to siphon detritus and other noxious material off the bottom where they accumulate during a water change.

There is not a lot of rocket science in doing a water change, but there are some techniques that can make it even more advantageous. First the powerheads should be turned off and the current in the tank reduced. Then, with a bulb baster, any detritus or waste materials can be blown off the live rock and corals. By reducing the current in the tank, the detritus that is present and that is blown from the nooks and crannies will settle to the bottom of the tank. Once the detritus has settled it can then be removed during the siphoning of the bottom.

The reason that it is important to get the detritus out of the tank is that the detritus acts as a nutrient sink from which algal blooms may get their nourishment. Organic phosphate, nitrate, algal spores, etc., are all present in the detritus that accumulates in the spaces in the corals and between the live rocks of every tank. If it is allowed to accumulate to high levels the result will be the rapid growth of algae. The reason for this is that algae are much more efficient at using these end stage waste products than are the corals. As a result when these nutrients accumulate the corals are at a disadvantage in that they are better at utilizing primary waste products, ammonia, fecal pellets, etc. In most areas on the fore reef, the corals are constantly bathed with water low in nutrients. However their bath is so large they are able to take up enough nutrients to sustain themselves. In the lagoon areas more nutrients accumulate and as a result animals from these areas can tolerate higher nutrient levels. This is why corals from these areas are typically good candidates to be the first inhabitants for a new reef hobbyist.

During a water change, the water and detritus should be drawn off and either flushed directly down a drain or drained to buckets lower than the tank so that an equal amount can be replaced. Clean new water should then be poured in to replace it. This water should be mixed so that it matches the old water in terms of temperature, salinity, alkalinity and pH. The match does not have to be precise, but the closer it is to the tank's water the less shock that is produced on the animals.

It is not enough to just match these characteristics however. There are two other factors that are often overlooked. First the water that is used to mix with the salt should be of the purest quality possible. I strongly urge anyone keeping a reef tank to use either reverse osmosis and/or deionized water for all of their make up water. It has now been my unfortunate experience to see several tanks fail when everything else was done right, because they used city tap water that had high nitrate and phosphate levels. Amazingly in these tanks, unlike any other that I have seen, the corals looked much worse after a water change than they did before one was done. In addition, algae seemed to proliferate faster after one of these changes. That is why I strongly advocate the use of good water purification system, and not just running the water through activated carbon.

The other factor that needs to be considered is that freshly prepared synthetic seawater is caustic in nature and will irritate invertebrates if added before it is given time to cure. By this I mean freshly made synthetic seawater should not be added to a tank, but rather it should be made up at least two days prior to a water change. I usually mix it up three days ahead and run an air stone in it prior to a water change. The most dramatic evidence that I have seen as to how caustic fresh seawater is occurred when I unexpectedly received an extra two hundred pounds of live rock during a shipment. Since I did not have room for this rock in a tank that I had prepared, I put the extra rock in an empty tank to which I added seawater that I had to prepare right then. The water matched all of the characteristics of the water in the prepared tank, except it was freshly made and then added to the tank. Amazingly the next morning when I went to inspect the tanks all of the rock in the prepared tank, where the water was five days old was in good condition. The rock in the fresh seawater tank however was quite different. Virtually all of the life on the rock, including the coralline algae, had been stripped off.

In order to ascertain whether doing water changes was essential or not I set up two 20 gallon reef tanks that were pretty well matched in terms of set up and inhabitants. One received regular monthly water changes while the other did not. For the first 6 months there was not an appreciable difference between them. However after 6 months and until the end of the experiment there was a noticeable difference. The corals did not open as fully in the non-change tank. Algae were more of a problem, particularly hair algae on the rock and microalgae on the front glass. Also it just did not look as healthy. Although this was just a single tank, it convinced me that a small regular water change was a good thing.

So having realized that a water change was necessary the next question was how often and how much water needed to be changed. Over the last two years I have been experimenting with just this question. In two tanks I tried to determine what is the most cost effective way of doing water changes. Both of these tanks were Berlin style reef aquaria so the conditions were at least similar. On one tank I started out doing one 10% water change every other month and on the other 10% per month. Once again after six months it was easy to see a difference. In the tank where only 3 changes had taken place more microalgae was present and in some spots slime algae was starting to grow, while in the once per month tank little of this was seen.

Then, to see if more was better, I then began doing 25% changes in the tank to which 10% changes had been done, while in the bimonthly tank the changes were increased to 10% every month. It surprisingly took only about six months for the 'no change' tank to catch the '25% change' tank, but after this first year these tanks were doing about equally as well. Surprisingly, after the second year, there was still little difference between the tanks. This was vindication that, with the methods that I was employing, the tank needed to have only monthly 10% water changes to thrive. This also confirmed the anecdotal reports of a 10% water change per month as being all that is necessary for most tanks.

The only time that I have found it necessary to do larger water changes is when something has seriously gone wrong in one of my tanks. Things such as a large anemone dying or one of the kids dumping in food come to mind or when I have gotten lazy and have not done a water change for a while. In these instances a large water change is definitely a good idea. When the animals have been stressed out by these conditions a large water change has almost acted like a tonic for picking them up. I have never had to do more than a 50% change (i.e. when a Sea Apple got caught in a powerhead) but my feeling is that more than this would probably be just as stressful as the problem that caused the stress in the first place. In conditions other than this, the 10% water change once per month is all that is really need to do if you are using a proper method of filtering your tank that includes good protein skimming, mechanical filtration, carbon etc.

Mike Paletta is the author of The New Marine Aquarium and Ultimate Marine Aquariums. He has been in the hobby for over 15 years and has written numerous articles for Aquarium Fish Magazine, Tropical Fish Hobbyist and Aquarium Frontiers.