(if this is in the wrong area, feel free to move)
Hi everyone, I have a question about kH,
I have a 20 gallon fresh water aquarium. We have plenty of african dwarf frogs, kuli loaches and mystery snails in our tank. However, our kH constantly changes. Some days we only need 3 drops to get the color change to happen in the test tube, other days we use 12 drops (the max number of drops for the test kit) and no color change happens. This seems to happen every other day. No one in the tank seems to be acting abnormally though. The loaches swim along like usual, the snails "crawl" across the class and cling to the plants, and the frogs do their usual froggy thing. Is this normal for the kH to fluctuate so much? Nitrite, ammonia, and pH remain relatively steady though (ammonia only changes .25 from day to day, nitrite remains at 0 and pH remains at 6, we have really hard water, so it's difficult for the pH to change)
The chemistry behind an alkaline buffer (KH and GH) is quite complex dominated by the carbonate ion, CO3 --. Carbonate minerals are extremely varied and ubiquitous in chemistry. The most common are calcite or calcium carbonate,
Carbonates are be used by plants for photosynthesis in case of high light when carbon dioxide (CO2) is limited or absent. They can just as easily get their C atom from CO3 -- as from CO2. So what happen to the negatively charged O atoms, they react with hydron (H+) and the ions of electrolytes, sodium (Na+), potassium (K+), calcium (Ca++), magnesium (Mg++), you starting to see a pattern here. We talk about the nitrogen cycle in aquarium so much in he hobby but totally forget about the carbon cycle in freshwater aquariums. The salt guys have a better handle on it than we do.
Are you suggesting that, once the CO2 is used up by plants, they start using CO3 and leaving that extra O- to oxidize or join those mineral ions (IE: Mg++)? (They put out O2 during the day anyway, yes?) I can see why that would lower KH/GH. But the OP's set-up shows a large variation.
Does having lots of plants and high-intensity lighting routinely lower KH? Is this a reliable method? There a re many of us on here with that problem
Just when I think I'm getting a handle on it, it gets more complex and abstruse. Thanks, Rick.
In nature everything seeks a balance.
And CO2 is the most soluble of the atmospheric gases in water so it would be impossible to have CO2=0 but yes in a shortage of CO2 the plants will use the carbonate lowering kH and yes the put out O2 but that is not ionic O and on the flip side if the plant are using nitrate or nitrite as opposed to ammonium as a nitrogen source they are rising kH
And when you think about what do kH and gH test kit measure the carbonate and medals in the water No they measure cation (+) and anion (−) in the water. So yes just when you think you have it ..........
As I understand it plants preferentially use ammonium which they derive from ammonia, ammonia which can be derived from nitrite or nitrate....or however that's done. So, if you can keep the plants using only ammonia and keep them using up all the CO2 and then starting on the CO3, then you can lower KH/GH.
Hmmm..... But this seems to be working at cross-purposes. I guess relying on plants to soften the water is a tricky balance thing, and not reliable in practice.
In all is hypothetical decussation we never really answered any questions. I would suspect with the pH at 6 the actual kH is low, in the 50 ppm range. The varying test result is most likely changes in the chemistry that are related to this being a plant tank. There are thing we don't know, how much light, what the substate is, how heavily planted, the bioload, etc. so coming to any informed hypotheses is unlikely. you would need systematic testing over time before drawing any conclusions.