How to Use Kalk Safely and Effectively in Your Tank

Kalk is king in the saltwater tank additive world. For you reef enthusiasts and reef junkies, its a must for your tank.

However, it is very potent and can ruin your whole tank if you don’t follow these steps:

#1: NEVER add it directly to your tank.

Kalk has to be dissolved in RO/DI water, NOT added directly to your tank water. If you add it directly to your tank, it will precipitate calcium immediately which means your tank will look like its snowing. Not good.

#2: Use it in your top off reservoir not a kalk reactor.

When you add kalk to your top off water in your reservoir, then you can adjust the concentration of kalk going into your tank. If there is too much kalk in your reservoir such that your tank’s pH and Alkalinity are going up too much, then you can add more RO/DI water to dilute the concentration. If it isn’t raising your pH/Alk enough, add more kalk.

My issue with kalk reactors is that they dump in 100% concentrated kalk to your tank. This is a problem during the summer when you are toping off a lot due to the heat. More top offs with concentrated kalk means more drastic pH/Alk swings, which means your corals won’t be happy. I helped a reefkeeper who had a kalk reactor and his Alk was swinging between 8 and 10 during the course of 12 hours. This is WAY too much and his corals started bleaching out because of it.

Plus, kalk reactors can be expensive. You’ve already got your auto top off setup (if you haven’t, you need to ASAP) so that part of your tank is already paid for!

#3: Start slow and increase the amount you add slowly.

Every tank responds to additives differently so when you are starting to add kalk to your tank (remember to add it to your reservoir, not directly to your tank), you’ll want to start with a low concentration – 1 teaspoon/gallon – to start. Run that concentration for a couple of days and see how much your pH/Alk swings. You want your pH to rise no more than .02 whenever your auto top off (ATO) runs. So if your pH is 8.00 when your ATO kicks on, after it kicks off, you don’t want your pH to be above 8.02.

Increase the amount of kalk in your reservoir until your pH rises more than .02, then back off 1 teaspoon/gallon. Keep note of what level your Alkalinity stays at as this level will be your “baseline”. Any reading above or below it and you’ll need to revisit how much kalk you are adding to your reservoir.

#4: Seasonal changes will affect how much kalk you add to your reservoir.

During the summer months, you will use more water from your ATO reservoir, so you’ll want to decrease the amount of kalk to you add. In the winter, the opposite will happen. You’ll use less water from your reservoir, so you’ll want an increased concentration of kalk.

Having your baseline Alk values in place outlined in #3 will let you know how much kalk to add/not add.

For my tank, I add 4 teaspoons of kalk per 5 gallons in the summer and 7 teaspoons per 5 gallons in the winter. My Alk stays @ 8 year round.

I cannot say enough about kalk and how much it keeps your tank’s levels in check. Like any potent tool though, it must be used carefully. If you start slow, then you can’t screw it up. Add too much too fast and your corals will bleach and your tank will suffer.

Use kalk wisely and watch your tank will thrive.

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Comments for this article (40)

  • pbnj says:

    Mr. SWT, if you could, please address the issue of keeping the kalk ‘dissolved’ in your top-off container. Kalk stirrers do just that, so why is this not necessary if you add kalk to a top-off container?

  • Kalk stirrers stir the kalk so that as new water comes in, so that the kalk dissolves into solution.

    When putting kalk into your top water container, you are setting the concentration and not adding in any more fresh water, so you don’t need a stirrer. In fact, its better that you don’t stir the container as some solids will form on the bottom of the container and you don’t want those getting into your tank.

    Kalk doesn’t fall out of solution as much as it reacts with the CO2 in the air around it. However, it has been shown that rate at which this reaction happens is insignificant to affecting the potency of kalk as Randy Holmes-Farley is quoted in his article:

    “Limewater can lose potency by reacting with carbon dioxide in the air, forming insoluble calcium carbonate…The rate at which this happens in large containers, such as plastic trashcans with loose fitting lids, is much less than many aquarists expect. There is, in fact, little degradation under typical use conditions. Consequently, the dosing of limewater from such large still reservoirs can be just as effective as dosing using any other scheme, and may have substantial advantages.”

    My top off reservoir is a 5 gallon jug that has to be refilled every 2.5 days, so I’m not even the least bit worried about any loss of potency of the kalk.

  • pbnj says:

    So, you’re currently adding 4 teaspoons of kalk per 5 gallons of RO/DI water. After 2.5 days, are you emptying/rinsing out whatever is left in your 5g jug and starting with a fresh batch of kalk/RO/DI solution?

  • That’s correct. I dump out about the last inch of water, rinse out the jug, reload it with kalk, refill it with RO/DI, shake it as it’s filling to get the kalk to dissolve, then I place it next to my tank.

  • Steve says:

    What top-up device/method are you using?

  • I have a 5 gallon jug for the reservoir, a Bulk Reef Supply 50 mL/min pump, and an double float switch.

  • Steve says:

    What pH are you hoping to maintain in the tank? I also presume you are using a digital pH monitor if you are about to measure a change of 0.02 pH?

  • tank’s pH varies season to season and I don’t aim for a set pH. In the summer I run 7.7-8.0 and in the fall/spring, its 8.0-8.3. Winter, back to 7.7-8.0.

    I am using a controller with a pH probe to monitor pH changes that small. They wouldn’t show up on a test kit and test kits drive me insane!

  • Steve says:

    I now have a pH probe and the pH is going between 8.1 during the day and 7.99 at night. The tank has soft corals and a few LPS. At the moment I don’t dose Kalk. Do you think there would be any benefit to me doing this, or should I just leave it alone as I don’t have SPS?

  • Ryan Reeves (Servo) says:

    I would really advise a Kalk reactor instead of putting Kalk into a large reservoir. It is very easy to have a float swtich get stuck or become dysfunctional dumping your entire reservoir into your tank. Many aquarists have lost there entire tank to this. Just think about your skimmer overflowing and draining your top off reservoir.

    THe key is have a Kalk reactor that fits your tank. If you are have such significant Alk/Ca swings, you are adding to much for the size of your reactor. Safety is in redundancy. Kalk is an incredible way to keep a gorgeous healthy tank, but it’s not without risk. Using a reactor that is properly adjusted mitigates those risks!


  • Kevin J says:

    Just got Kalk in added to my ato last night. Thanks Mark

  • Ryan…you are correct that by putting kalk in your ATO, there is the risk of a float switch getting stuck open. However, the same risk holds true with a kalk reactor fed by the top off reservoir. If the ATO get stuck on, then the top off reservoir with drain, taking all the kalk in the reactor with it. I’ve seen plenty pictures of all white tanks (all the coral died) b/c the reactor got stuck open.

    Either way there is a risk. I personally wouldn’t want to introduce another place where something could go wrong by adding a kalk reactor that is driven by a pump etc.

  • Great! Let me know how it goes.

  • Bryan says:

    Mark, I know you did this video a while back but if you’ll indulge me for a moment…. I was just curious if you can see that line where the water is “milky” vs clear through that white 5 gallong jug. I have the same ones and wanted to make sure I could see that level to drop the ATO line just above it. (This line is mentioned at 7 minutes into your Kalk video in case I’ve confused you.)

    Lastly, is all your top off from Kalk water or is it ever just RODI and only sometimes Kalk?

    Thanks for your time.

  • Bryan…the line is about 1″ from the bottom of the jug.

    And, the top off water always has kalk in it.

  • Adrian Brown says:


    I hope you’re doing well. Thanks for all of the great info you put out for everyone. I have a question for you? I have a 75 gallon reef tank. Can I be successful doing frequent water changes and using a part A and B solution and test frequently? Will my coral do well?

  • Lucas says:

    I have heard that ATO switches fail and cause a kalk overdose. What can be done to prevent this?

    Could one set up a dosing pump on some mixed kalk seperate from the ATO?

    Why aren’t you using the aqualifter pump anymore? (In one of your responses you said you are using a BRS dosing pump for ATO)

  • Lucas…any switch can fail and I let my APEX control the float switches so if they do fail, the Apex will handle the situation.

    You could setup a separate reservoir for kalk, but if you are dosing kalk, then your ATO will probably never run so you might as well run Kalk in your ATO.

    I dropped the aqualifter as it was leaking.

  • Lucas says:

    What is needed to set up the APEX that way? So many uses for the APEX! 🙂

  • Lucas…not sure what you mean…in what way?

  • Lucas says:

    Sorry for being vauge, is there certain equipment needed to set it up to control the float switches.

  • Lucas…The breakout box can control the float switches.

  • Curtis says:


    I’m new to the hobby and can proudly say that your videos convinced me to make the conversion to a reef tank (enthusiast).
    Anyway, I wanted to clarify something: Shouldn’t you start out at something closer to 1tsp/5gal, not 1tsp/gal like it says in #3? I think your video on this is correct, but I wanted to make sure that people were getting the right info.

  • Robert says:


    I have been looking into switching from 2-part to Kalk and I initially intended to run it through my ATO. However, I have had several reefers inform me that kalk will 1) clog up the ATO line requiring more frequent maintenance, and 2) kill the ATO pump. What is your experience with this?

  • Robert…kalk will kill cheaper ATO pumps. I blew through an aqualifter pump a month until I stopped being cheap and got a Bulk Reef Supply 50 mL/min pump. Once I got the bulk reef pump, I never had a problem again. Also, the ATO line will clog only if you let the line drain out completely such that the kalk in the water dries up.

  • Robert says:

    Thanks Mark.

    Do you consider most magnetic drive pumps to be cheap? Would a peristatic pump work better than say, a rio600? My ATO reservoir sits behind the tank/sump in the fish room, so smaller ATOs don’t have the horsepower to make the trek.

  • Robert…some of the magnetic drives are well built like Tunzes and Water blaster pumps. That being said, I wouldn’t use a magnetic drive pump to dose chemicals into your tank. The flow rate is too fast. A peristatic pump would likely get the job done for you despite the distance.

  • Nick says:

    Just started adding kalk in my ato, put 1tsp in a 5 gal jug and I noticed today (2days later) the water in my 5gal ato is cloudy. Should I be concerned that the water turned cloudy? It was clear at first and I am using ro/di water. Not seeing any negative affects in my tank.

  • Nick..the cloudyness is normal

  • Rick Vignery says:


    I am a little confused on your point #3.

    “You want your pH to rise no more than .02 whenever your auto top off (ATO) runs. So if your pH is 8.00 when your ATO kicks on, after it kicks off, you don’t want your pH to be above 8.02.”

    Everything I have ever read on this from people like Randy Holmes Farley for example states that your P.H. can change by up to .2 with additions of Kalk without ill effect. So 8.00 can go to 8.20 then. It seems to me that it would be almost impossible to add anything that raises PH at all that would not quickly raise the level past .02. Am I missing something here?

  • Tony N. says:

    Thought I would put my $0.02 in here.

    I have just started using Kalk+2 and like others have mentioned, I was VERY worried about something happening to the tank and losing everything that I have worked hard to build up. A little relief in this is the Apex tank controller. As Mark stated in his video, a tank controller is invaluable.

    In my tank, I have it set so that the ATO will only run for a certain amount of time, and if the float sticks, it will send me an email letting me know there is something wrong. I also have a failsafe built into the outlet for the ATO pump where if the PH goes above 8.30, it will not turn on, and again, I will get an email. It’s a simple code and it can save your tank. [If PH < 8.30 Then Off].

    So do yourself a favor, if you are going to automate dosing, get a tank controller and put as many failsafes as you can in there to keep from cooking a tank.

  • Rick…you are correct that I’m not wanting a pH raise above .02. Whenever I’ve raised my pH more than .02, my SPS barked at me by bleaching.

  • Kirk k says:

    Good morning mark

    I have a 90 gallon fully established reef aquarium and in the entire 1 year history of my tank i haven’t been able to increase the ph above 7.96 instead of the original media in the back canisters i removed the bio balls put in a filter sock. And have the entire apex system monitoring my tank. If u could contact me I’d like to discuss what steps i can take and how I could go about stabilizing it.
    Thank you.

  • louis says:

    Does the top off tank need to be cleaned out after every use? I personally never clean because its always in use. 5 gallon fills as needed and I just mix a half ounce of kalk and manual mix. The ato energizes every 4 hours for seven minutes and fills an IV bag that drips it in slower. Never saw the need to wash out.


  • Samich Saweeng Batta says:

    Mark, can you please update your opinion on this matter? -especially since this was uploaded in Oct 2010, and then 2 months later, you uploaded your new Apex tutorial. Ladies and gentlemen- Kalkwasser can be controlled much better in 2016. Putting Kalk in the top off water has a lot of draw backs.( pump corrosion, concentration build up, the mess and more…) Kalk in your ATO, IME: is now classically labeled as “Old School”.

  • Samich…I still use kalk in ATO with no problems with pumps. I use the Tunze osmolator now and have never had one of their pumps die even when sitting the in kalk sludge.

  • Emmanuel says:

    Hi Mark,

    I just installed a 28 gallon barrel filled with RODI water running with a Tunze Osmo 3155 . My system is only 33 gallons in total (Display tank & Sump).

    I have issues with my pH as I constantly open up my windows and up here in Canada at -5 degress and below I can’t always open them and the owner downstairs is not happy with it. 🙂 Well, my pH goes in the morning from 8.03 to 8.21 around at the end of the day.

    Once I exhaust the water in my 28 gallon barrel, I was planning on starting to dose it with kalk. How should I go about this? Like fill it up with exactly 28 gallons of water and then dose it? I’d appreciate if you could guide me more regarding this as I don’t wanna end up with a disaster with my first tank.

    Continue the great work!

  • jacob Liawatidewi says:

    Question on which part of the sump that best to dose the kalk ato. Like most sump, I have 2 areas that separated by baffle, the skimmer area which maintain the level and the return pump area which will have lower level with evaporation.
    I have my ato censor in return pump area (using the tunze osmo). If I dose the ATO in the return area, it will allow the censor to quickly read the make up water level and shut off the tunze, but I don’t have any water movement in this area which I worry with the build up in kalk.

    I can dose in skimmer area which have lots of water movement, but will the censor able to capture the raise water level in timely manner?

    Thanks for the feedback

  • Nathan Taylor says:

    Is it absolutely essential for the ATO reservoir to be sealed if using kalk? I currently use an old 15 gallon tank as my ATO reservoir with the tunze osmo pump discharging into my return pump chamber and was thinking about starting kalk. It is a 180 gallon mixed reef and I do have the apex gold controller so safety isn’t really an issue for me. I’ve read that once the mixture sits long enough in the reservoir it can develop a layer on the surface that can protect it from breaking down. Others say that it must be kept in a sealed reservoir. Thoughts?

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