Beginner Series #2: How and Why Do Water Changes Work?
What Happens If They Don't Work?

Or How to Attain Majchrzak Status

This article is to provide beginners with baseline knowledge of why and how water changes work and when they fail in restoring critical water parameters, so that dosing alternatives might be employed. I will also address many misconceptions about water changes.

Image 1: Junior's Reef by Gary Majchrzak. A system that the author has always emulated.
Photo is used with permission from Gary Majchrzak, ©2019, All Rights Reserved.


We all give the same advice. If you are new to the hobby, almost every person you will meet, every local fish store you will visit, will repeatedly tell you to do water changes. A recent article R2R article by Cynthia White highlighted many aspects of why and how to do water changes (White 2019). All of us initially follow that good advice without really understanding why.

As Cynthia pointed out, we do water changes for many reasons. One primary reason is for removing waste chemicals like nitrates and phosphates. This is important in all tanks, but it is especially true for a new tank as it is being established.

Another important function is for replacing crucial water parameters, such as alkalinity and calcium, that are utilized by corals as they grow. The amount of corals in your tank and their rate of growth will eventually be limited by your ability to replenish the alkalinity and calcium in your tank.

As your tank matures, its needs will change. Your ability to identify those needs and pivot your husbandry approach, will greatly enhance your success.

With water changes being by far the most common method employed for water parameter management, it is important to recognize the signs of change and the need to modify your strategy.

We all have the same ultimate goal. What drew you to the hobby was a visit to the aquarium, a party at a friend’s house with a reef, or perhaps a visit to a local fish store where you became mesmerized by a show-stopping reef tank with walls of colorful corals.

Upstate New York, Rochester in particular, is home to many of these tanks. My goal was always to have a tank like Gary Majchrzak. His first tank that I saw, Junior’s Reef, was nothing short of drop dead stunning (Figure 1). He is one of the local elite, humble, reefing aquarists who creates unequivocally beautiful tanks in our community. And Gary has another enviable quality to go with his reefing skills: a laid-back, relaxed attitude as in "Testing? What testing?"

Hopefully, someone like Gary lives in your community. Like Gary, the reefers that manage those tanks have undoubtedly all mastered the process of effectively replenishing the alkalinity and calcium as fast as the corals are using them. Make no mistake--it is fast, very fast (more on that later).

If you rely solely on water changes to solve this problem when you have that wall of corals in your tank, you are going to be a very unhappy camper.

Here’s why. Let’s start with why we all hate water changes.

My love-hate relationship with water changes. When you bought that 100-gallon tank and someone told you that you would have to do water changes, you probably didn’t even blink an eye. Neither did I. Don’t get me wrong, I appreciate the value of water changes, but here is why I hate them just as much.

“As a general guideline, a monthly 25% water change is recommended. However, smaller, more frequent water changes (15-20% every 1-2 weeks) are preferable for heavily stocked aquariums or aquariums with large fish.” (LiveAquaria)

No one probably told you that this is expensive and physically demanding. If you change 20% of your water every week, you will change 1,024gal/yr for a 100-gallon tank. If you have a 30-gallon sump, that increases to 1,352gal/yr.

When buying the tank, the store likely also encouraged you to use a reverse osmosis unit for removing dissolved solids from your home tap water (Hunt 2013). What they didn’t tell you was that the RO unit will waste about 4 gallons of water for every gallon it produces (Timmons 2011). That means you will draw 6,760gal/yr from your home water system.

In addition, you will have to buy seven boxes of reef salt per year (200gal/box) for that 1,352gal of water. At $63/box, that is $441/yr (BulkReefSupply). (Did I hear you say that you bought a 200 gallon tank, not a 100 gallon tank?)

It’s seems to always be a 20 -year-old that tells me it’s no big deal to do water changes!

In my case, I put the RO unit in my basement. Saltwater weighs 8.5 lbs/gal. If you have to lug 26 gallons of new water up the stairs and 26 gallons of old water down the stairs, you are lifting 442lbs/week. That is only 22,984lbs/yr!

When I started this journey, I never did that math. In all honesty, I had to check the calculation a few times before I believed it. Let’s face the facts, this is tolerable for a while but it gets very old in the fifth year. The main reason people eventually leave the hobby is either because of the physicality of conducting water changes, or because of the consequences of not doing them. As I near 60, I absolutely have no vision for lifting 10.5tons/yr of water up and down my basement stairs. For me, something had to change or I was going to eventually quit. (Really – you bought a 200 gallon tank?)

I vividly remember the day I visited a friend whose family owns a local fish store, and I told him that I was going to pursue a Balling method (Balling 2008) to eliminate my need for changing water in my next tank. He told me that I would be chasing water parameters and wasting time on chemical tests when I could have simply changed the water.

That was a hard blow from a valued colleague that I would think about for months as I began my next build. What I learned throughout that journey was where I really missed the boat in doing water changes with my previous tank. Remember, there is a time and place for everything!

Why my last tank never reached “Majchrzak” status! If you couldn’t tell, I really hate water changes with a passion (almost as much as Vortech users hate wires in the water!) Consequently, my strategy with my previous tank went like this. I triggered my water changes on visual observations such as: 1) how dirty the bottom looked, 2) how much algae grew on the side of the tank, and 3) if the corals just didn’t look happy.

Occasionally, I would do a water test, and if it looked off, I would do a bigger water change. This strategy sustained the tank, but it never seemed to get better. The next strategy I tried was to change 20% of the water every week regardless of how the tank looked. While this worked better, I still never managed to achieve consistent success.

My final effort was to use a method of dosing 2-part Bionic solutions made by ESV (ESVAquariumProducts). That worked even better, but I still felt like the tank was underperforming. I never did make “Majchrzak” status with that tank. When it crashed after refinishing floors, I knew it was time for a change.

As fate would have it, Mr. Majchrzak and I started our latest builds almost at the same time. We both took on the goal of no water changes for almost the same reasons. It wasn’t a competition, but at least I knew our bottles of “wine” were starting to mature at the same time and rate. It certainly helped to calibrate my progress.

When you set up a tank with a no-water-change methodology, the “why” and “when” of water changes becomes readily apparent!

The “why” and “when” of water changes. There are numerous common elements of natural sea water that are important in a coral’s overall health (Borneman 2008). Amongst the top seven water parameters described as either essential or critical are alkalinity, calcium and magnesium. The four other elements are salinity, temperature, oxygen, and phosphate.

Alkalinity and calcium are important because they underlie the calcification process of corals. We often ignore this when starting a tank because the waste products produced by our fish are far greater (and far more important) than the utilization of alkalinity and calcium consumed by the few small corals we own.

In a new tank, your water changes will serve to remove the waste products as well as replenish alkalinity and calcium consumption. When your corals grow, they will consume more and more alkalinity on a daily basis.

Figure 1 shows just how fast alkalinity changes can occur in an established tank. In my tank, the alkalinity dropped from 8.12dKH to 6.3dKH over 3 short days after I stopped dosing Aquaforest KH solutions. If I relied on water changes in order to maintain alkalinity values near 8.0dKH, it is likely that I would have to change water multiple times each week. Clearly, water changes are not the solution for me at this point in time. As your tank matures, the big questions will become: 1) When will my water change strategy fail to replenish calcium and alkalinity? 2) How can I tell it is happening? and 3) What can I do about it?

Screen Shot 2019-01-19 at 9.25.31 AM.png

Figure 1 was created by and is used with permission from Dr. Greg Gdowski, ©2019, All Rights Reserved.

Let’s run through an example:

Consider the scenario illustrated in Figure 2A. A small amount of corals in a new tank will only consume small amounts of calcium and alkalinity. In this circumstance, your initial consumption rate of alkalinity over time will be small (blue line, Figure 2). Regular 20% water changes every few weeks (red line, Figure 2) will readily restore your alkalinity within the ideal range (yellow highlighted area, Figure 2). At this point in time, there is no need to augment or supplement calcium and alkalinity in your tank. I call this phase a state of happy misguided bliss.

I started this article by saying we all have the same ultimate goal. You will add corals, many corals, likely as many as you can afford (it is an addiction after all – didn’t we tell you that?). Much like an addiction, your desire to add corals will gradually increase your alkalinity consumption (green line, Figure 2A). Eventually, your regular 20% water changes will no longer restore your water parameters and the alkalinity will decline below the ideal range. You should have been testing alkalinity levels all along, nonetheless, the days of test-less bliss are now over if you are to succeed.

Screen Shot 2019-01-19 at 9.27.27 AM.png

Figure 2 was created by and is used with permission from Dr. Greg Gdowski, ©2019, All Rights Reserved.

You are now at one of the first major milestones in your journey. You have come to a fork in the road. This is when you will transition from a beginner-level reefer to an intermediate-level reefer. If you pick the wrong side at the fork in the road, your corals will stop growing and it's possible some of them will not thrive at all. Your addiction will force you to continue to add corals, amplifying the problem further, causing you greater frustration when nothing thrives.

Solutions to the problem of water parameter degradation. At this juncture, you really only have four potential solutions to the problem.

1. Fragment your corals and reduce the need for constituent elements.

2. Increase the amount of water changes or the frequency of water changes.

3. Start dosing constituent elements.

4. Migrate away from SPS or LPS corals to soft corals such as zoanthids or mushrooms, which don't use calcium and alkalinity.

Option #1 is akin to telling a smoker that they should quit. It’s hard to fragment corals that are not growing in the first place. In order to solve the problem, you will have to remove those fragments and put them in a different tank. Alternatively, you will have to stop buying new corals all together.

Option #2 is a solution based on manipulating your water change schedule. This solution works, but it should be thoughtfully executed based on chemical testing. Your first inclination will be to stay on your same schedule, but to increase the amount of water changed out (Figure 2B). Unfortunately, in doing so, you may have created two problems. 1) If your consumption rate is high, your water parameters may still decrease below ideal values before your next water change. 2) The water parameters will exhibit higher variability over time. Long-term success is all about stability. The best strategy, unfortunately, is to increase the frequency of water changes (Figure 2C). This strategy maintains values within an ideal range. In addition, it produces values that are higher on average that exhibit less variability over time.

How do you know when the solution is working? The only way to check to see if your water changes are working properly is through water testing. Test your water twice: 1) test it just before your water change, and 2) test it again about 2hrs after doing the water change to ensure that your new water has been thoroughly distributed.

A. If both tests yield low values, then you are in the scenario depicted in Figure 2A. This is by far the worst-case scenario, because your problem will only get worse until you remedy the situation. More frequent water changes are justified.

B. If only the first test is low, then you are in the scenario depicted in Figure 2B. This scenario is tolerable but is less than ideal. This likely means there are time periods when your corals are underperforming in growth and are not looking as healthy as they could. Similarly, more frequent water changes would be justified.

C. If first test is lower than the second test, but both are within the ideal range, then your water changes are working effectively as depicted in Figure 2C.

For me, if you haven’t figured it out already, this is where my true “hate” of water changes began. The use of water changes to correct this problem never gets better. You start off with water changes once/month. Then it's twice/month. Eventually, it becomes once/week. I think you get it.

More and more weight being lugged up the basement stairs. From my wife’s perspective, that meant more and more water spilled on the floor. From my perspective, that meant never building a much larger tank (ie. the second goal of every reefer) in a different room with a wood floor. In any case, for both of us, something had to change.

A number of years back a few reefers built some of the first DIY automatic water changers. Today, there are a number of automatic water changers on the market at reasonable costs (AutoAqua). The primary advantage of these systems is that they provide more stability and less variability in water parameters over time. This is accomplished by reducing the amplitude of the water change (red line) but doing it more frequency (perhaps every hour instead of every week) as in Figure 2C. It is a solution worth considering. The primary disadvantage is that two reservoirs have to be utilized to hold and maintain waste water and fresh salt water.

Option #3 is to start dosing constituent elements. This will be the topic of my next article. Once you head down this path and have mastered it, you are really halfway to a no-water-change solution. Indeed, it was the implementation of these options that gave me the confidence to go the whole distance in using a Balling method (Balling 2008) to altogether eliminate water changes in my current tank.

Option #4--self-explanatory.

Until next time,

Happy reefing!


What is Majchrzak status? It is when your local fish store asks you to answer their phone when they are overrun with business!

Table 1, adapted from the Public Aquarium Husbandry Series of the Royal Burgers' Zoo shows the general ranges of calcium and alkalinity in comparison to natural sea water (Borneman 2008).

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AutoAqua. "Auto Water Changer plus automatic topoff system." from

Balling, H.-W., Janse, M., and Sondervan, P.J. (2008). Trace elements, functions, sinks and replenishment in reef aquaria. Advances in Coral Husbandry in Public Aquariums. . R. J. Leewis and M. Janse. Arnhem, the Netherlands, Burgers’ Zoo. 2: 143-156.

Borneman, E. (2008). Introduction to the husbandry of corals in aquariums: A review. Advances in Coral Husbandry in Public Aquariums. . R. J. Leewis and M. Janse. Arnhem, the Netherlands, Burgers’ Zoo. 2: 3-14.

Bulk Reef Supply. "Instant Ocean Reef Cystals Reef Salt." from

ESV Aquarium Products. "B-Ionic Reef Keeping (BRK)." from

Hunt, P. (2013). Marine Aquarium Basics, Part 1: Water and Salt Tropical Fish Hobbyist.

LiveAquaria. "Importance of Water Changes in Aquariums." from

Timmons, M. (2011). "Is Reverse Osmosis Wasteful?". from

White, C. (2019). "Water Change FAQ." Reef2Reef.


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Author Profile: Greg Gdowski, Ph.D.

Greg has 20 years of aquarium experience, and he has been keeping reef aquariums for the past 10 years. He and his wife are also both dog lovers and have two special-needs Vizslas at home.

Greg is also the Executive Director of the Center for Medical Technology and Innovation and Associate Professor in the Department of Biomedical Engineering at University of Rochester.