Many of us are led to believe from such an early beginning, that in fish keeping, water changes (to our aquariums) are considered essential and so any such suggestion of them potentially being ‘bad‘ are met with hostility and confusion.

Are water changes really a good thing though? Can they even really be a bad thing?

Well before we can answer that question we really need to understand some reef keeping science first.

90% + aquarist, freshwater or saltwater probably keep fish of some kind. They feed their fish and thus the fish then  produces waste (the Nitrogen cycle). This waste is broken down from Ammonia to Nitrite to Nitrate. Now in most circumstances this is where it ends. An experienced hobbyist can achieve one step further and in anaerobic low oxygen zones they can convert this Nitrate to a dissipated Nitrogen gas which becomes harmless, but if we are honest, the vast majority of us will be left with Nitrate.

Nitrate is an unwanted nutrient that in significant numbers will harm coral in a reef tank eventually causing death and in vast numbers can harm fish to ultimately also causing them death.

Nitrate isn’t the only common unwanted nutrient, but Phosphate is also a by product of our feeding habits, again something we do not want. This too can cause coral death, not to mention fuels algae making our aquariums unsightly.

Now unlike the things we don’t want, our reef aquarium water contains something that we do want, and thats the elements. Whether you use natural sea water sucked straight from the ocean or whether you mix a synthetic salt with some RODI water, that salt will be comprised of various elements. The Salinty of the salt overall dictates how much water and how much minerals and elements are contained within, but 99.9% of these are beneficial and needed in our aquariums.

So water changes, do i need to do them? Well ultimately in any aquarium, stability is key. The most successful aquarium will be the one thats consistent and not necessarily the one with the best numbers.

Starting with those unwanted nutrients, some hobbyists will employ methods to remove them such as refugiums or algae beds, carbon dosing or bio pellets, excessively heavy skimming but to name a few. Now all of these things may work and may reduce the nutrients within our aquariums to desired and acceptable levels.

Alternatively, some people are unable to reduce these nutrients with such methods and they may need to complete water changes in order to dilute the bad stuff (i.e Nitrate and Phosphate). These water changes now become essential to maintaining a reef tank nutrients, but only if you were unable to do so by other means. If you’re removing nutrients by one of the other suggested methods and it works, why would you still need to water change?

Those elements we spoke about earlier, well corals utilise and consume them to not only grow, but to also survive. Without these elements our corals would simply perish and die. To them, these elements are what food and oxygen is to us, a lifeline. As corals within our reef tanks grow, they consume these elements. Think of it like humans and our homes, as we consume food within our homes, we grow, not always in the direction we desire, but we grow. When we run out of food at home, we need to go and buy more or we would begin to go hungry ultimately becoming poorly and in dire circumstances, die.

Well our corals are no different, we must replace elements that they consume through survival and growth to ensure that they continue to survive, grow and flourish. We can do this in a number of ways. We can either ‘dose’ additives to the aquarium via bottles of additives or via calcium reactors or we can conduct water changes to put fresh element rich water back into our aquariums.

Now baring in mind stability to any aquarium is key, the dosing option will be much safer and more preferential to any aquarium. We can establish how much of each element our corals consume per day and safely dose that element back into our aquariums, hour by hour, day by day. This means that any depletion is quickly repleted and levels are not able to fluctuate drastically.

Water changes too can replace these consumed elements, but particularly in larger more heavily stocked tanks, water changes are generally done once a week or maybe even fortnightly. This can mean that those elemental uptakes are only replaced once a week or fortnight allowing levels to drop drastically between changes. This means that when conducting water changes the large rise between old and new water can cause an unwanted swing further upsetting your corals.

Equally lets look at the most advanced reef keeping technology and to ICP testing. An ICP test will tell you what your aquariums parameters are. It will tell you if you have excess nutrient and/or whether your elements are at the desired level. Now if we assume the answer here is everything is good, you have enough elements as your dosing is keeping up and that your nutrients are low and stable as your export method is sufficient, can a water change cause damage?

Well if you already have stability and balance, doing a water change can indeed cause problems. It may cause a swing in the elements or nutrients and that can cause corals problems, it can actually kill them.

You may also, even with the best RODI units in the world, introduce unwanted elements like Silicates or heavy metals. Imagine spending all that money to get your aquariums parameters stable and within range, only to upset it and throw them out by an innocent water change.

Too often we can look to water changes as routine, without thinking is it actually necessary. In some circumstances, the answer is yes, we need to and must do water changes, but always and in every circumstance?

So are water changes really the best way of keeping a reef tank happy? Water changes are a fantastic tool for nutrient removal and for element replenishment. In an emergency or when things are going wrong they can be invaluable, particularly if you are unsure as to why, but for an experienced reef keeper or to those who have found that harmonious balance or stability and desirability are they really the best tool?

Next time you consider a water change, remind yourself of the above and consider, are you already replacing all of your elements sufficiently and are you able to control your nutrients? If the answer is yes, then do you really need to do a water change? If the answer is no, then dilution may be the solution.

As a lifelong adult hobbyist, making it almost 15 years in the making, Danny has been keeping Saltwater for quite some time. As one of the biggest passions in his life in 2014 he combined that with his second biggest passion, photography and videography.