FAQ #31: If stirring kalkwasser has negative effects, why are reactors available that do just that?

January 6th, 2017
brs.li/BRSkalk Today on BRStv, we have another episode of BRS 52 FAQ where we answer all of your frequently asked reefing questions from our popular 52 Weeks of Reefing series.

This week we are answering K Mam’s question: "You mentioned that we should not stir the solution after it has been mixed initially, but certain companies like Two Little Fishies have stirrers that work with ATO that stir the kalk when the ATO runs. Why is that?"

This is a pretty common question and at face value it is confusing because almost all of the advice out there tells you after you mix the kalk once, leave it sit, and do not mix it again. However all the kalkwasser reactors out there do that very thing; constantly mix the solution or at least do so many times a day.

The answer lies within how the kalkwasser is implemented in the aquarium, which in a vast majority of cases is mixed in a container and slowly dripped into the tank or added via an auto top off. In this case the kalkwasser or calcium hydroxide powder should be thoroughly mixed with freshwater once and once only and then sealed with a lid on the container to limit gas exchange.

Mixing it the first time allows the calcium hydroxide to fully dissolve into a solution of ionized calcium and and hydroxide ions. Once it’s fully mixed its often referred to as a fully saturated solution meaning there is as many calcium and hydroxide ions dissolved in the water as their can be.

After that, mixing it wont make any additional kalkwasser dissolve into the solution, in fact the opposite will happen. When you mix it you add atmospheric carbon dioxide into the solution from the surrounding air in the room. When the carbon dioxide enters the solution it reacts with the hydroxide in the water to form carbonate which in turn reacts with the calcium to precipitate out as calcium carbonate, reducing the available calcium and carbonate in the solution so it is no longer fully saturated. .

So simply put, mixing doesn't make the solution more concentrated it potentially makes it less potent and needlessly wastes raw material. The best thing you can do is put a lid on the container and leave it alone. It doesn't need to be a hermetically sealed lid , just one that limits the amount of fresh air inside the container. In fact, when left alone the saturated kalk solution will form a crust on the surface which essentially protects it from carbon dioxide exposure.

Ok, so why do all the reactors mix it constantly or several times a day then? The answer to that is fairly simple: they are small containers which are constantly fed new freshwater. With the previous solution you are often mixing up enough fresh water for anywhere from an entire day to week or even month.

With most kalk reactors they only hold enough water for a portion of the day and the nature of their design is they feed the saturated kalkwasser solution into the tank by pumping freshwater into the bottom of the reactor, forcing saturated kalk water out of the top. This means that freshwater that enters the bottom does need to me mixed. Since there is no air inside the reactor mixing it also won’t cause precipitation issues or dilute the potency.

So follow along as we answer today's question, provide some helpful insight, and help make reefing just bit more fun and easy for you and your tank.

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