Water chemistry question - little to no water change tanks

[Dawn T]

I've gotten a grasp on various elements of water chemistry, but one thing continues to elude me. KH. I keep seeing information that confirms that KH decreases over time in tanks that receive few or no water changes (topped off with RO or D water), but I haven't been able to find one that explains why. I know KH is a measure of the carbonate and bicarbonate in the water, but what uses that or otherwise causes it to decrease over time? I've certainly observed that with my tanks (my well-established tanks go months without water changes, topped off with D water to compensate for evaporation). I know RO and D water don't replace lost KH, but water changes do. I was using baking soda to compensate for lost KH in the tanks that didn't get monthly or weekly water changes but switched to crushed coral after watching one of Cory's videos. I'd just like to understand the how and why of loss of carbonate and bicarbonate. Anyone know?

[Streetwise]

It sounds like the answer is in the question if the top-offs are with RO or RODI water. I feel lucky with village water that is only treated with chlorine. I just age the water and top-off, with all minerals included.

[Dawn T]

But where do the carb/bicarb go? The Ca/Mg (GH) stays the same consistently, so those minerals stay in the tank. The carb/bicarb vanish, so they have to be going somewhere. Where do they go? Do they get used and not replaced by plants? Fish? Bacteria? Something else? They can't simply vanish in what is essentially a closed system of sorts. They have to be going somewhere, but I haven't been able to find out where.

[Hobbit]

I believe it’s mostly getting used by plants and algae, if this dude on Quora is to be trusted 😉 https://www.quora.com/Do-aquatic-plants-use-carbonic-acid-during-photosynthesis-or-do-they-dissolve-carbon-dioxide

[Koi]

15 minutes ago, Hobbit said:

I believe it’s mostly getting used by plants and algae

Yes the plants do eat away at kh over time and suppose algae does as well. Ive also heard that nitrifying bacteria uses up kh but I don't really know if that is true. If you never did water changes you will see over time that both gh and kh starts to lower which is kind of the idea behind old tank syndrome.

Also anything acidic will "eat away" at kh. So ammonia, nitrite, nitrate, and carbon dioxide will start chipping away at your kh if left unchecked.

[Will Billy]

From my research bacteria mostly consume carbonates. Plants also do too as every living organism on our blue dot called planet earth is a carbon based life form. Bacteria are the first ones to the smorgasbord, followed by fungi, then plants, then animals, all the way up the food chain. All life consumes and uses carbon wether directly as bacteria does or through eating things that consume carbon, such as me eating a steak that came from a cow, that ate the grass, that pulled carbon from the atmosphere in photosynthesis, that was produced by a dinosaur burp etc... carbon is the building block of cellular growth. If it grow in your tank it uses carbon. 

[Wes L.]

I can explain this. A build up in our tanks from the biological processes going on, such as the decomposition of organic waste, and the respiration process from both aquatic life and plants, the nitrogen cycle (biofiltration), and nitrates itself in high amounts, are all literally producing acids in our aquariums. All of the things I mentioned above can and do produce acids. And acids do exactly what we would expect acids to do....eat away at things. So all those acids "consume" the carbonates and bicarbonates. Of course acid doesn't actually eat the carbonates and bicarbonates, more accurately the carbonates and bicarbonates neutralize the acids by mixing with it and then produces a neutral salt (CaCI2) and a weaker acid (H2CO3) which disassociates quickly into CO2 and H2O. So bottom line is acids make the KH disappear through a chemical process....and the acids disappear in the same process. The carbonates and bicarbonates provide a very necessary "all-you-can-eat buffet" for the acids, because without the carbonates and bicarbonates, the acids wouldn't have their all-you-can-eat buffet, so they'd be going after the hydrogen and hydroxide ions, which of course is our PH...and we don't want that changing. This exact same process is something we are all really familiar with, especially after a spicy meal. Anyone who has taken Tums, or Pepto Bismol, or any of the other acid reducers for heartburn or sour stomach has participated in this exact same process. All those brands of acid reducers are nothing more than calcium carbonate....just like the carbonates in our aquariums. We eat (or drink) the calcium carbonate and then it neutralizes the acid in our stomachs.

[Dawn T]

Thanks much, gang! I had a feeling I could get solid answers from the forum. Glad I finally remembered to ask about it.