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I've got a pretty high KH of 15 (London water). I know boiling tap water in kettles can be used to remove temporary carbonate hardness, and was wondering if it would be viable to mix boiled water and tap water together in an aquarium to reduce the Kh levels and soften the water a little?
 

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I would say probably not, unless you can do that repeatedly and in a consistent manner every time you do a water change to keep parameters stable. Achieving that would be really hard, I think (as I'll explain); so it mostly sounds like a recipe for dangerously unstable KH levels to me. You'd have to let the boiled water cool down every single time, and you just can't be sure of exactly how much the boiling will lower your KH, how to make that amount standard, etc. (at least not without a lab environment)... Ultimately, I really don't think it would be worth all the effort. If you want a lower KH, the best option is to use RO water and remineralize it yourself. Second best would probably be using something like peat that you can easily use a set amount of and at least somewhat control.
 

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Probably not enough to matter. You're really going to have to consider getting a Reverse Osmosis filter as it's the only way to reduce the KH. The joys of living in the land of chalk.
 

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A quality RO/DI unit makes life much easier.
 

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Boiling water would remove carbonate by off-gassing CO2. That would shift the pH to more basic unless you provide another anion and would be immediately undone if you are trying to add CO2 for plants. Peat or other sources of acid do it in a closer to permanent fashion because they add other anions in the form of acids. You dont necessarily need RO. Distilled or deionized will have the same benefit. Dont know if they have them in london, but most grocery stores here have a water dispenser where you can fill 5gal jugs of "drinking water" that is softer than our taps.
 

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Boiling water would remove carbonate by off-gassing CO2. That would shift the pH to more basic unless you provide another anion and would be immediately undone if you are trying to add CO2 for plants. Peat or other sources of acid do it in a closer to permanent fashion because they add other anions in the form of acids. You dont necessarily need RO. Distilled or deionized will have the same benefit. Dont know if they have them in london, but most grocery stores here have a water dispenser where you can fill 5gal jugs of "drinking water" that is softer than our taps.
If you're referring to water softener, DI or de-ionized, it's not a great idea as you're replacing Calcium ions with Sodium, which tends to shift the water to an even more alkaline extreme of pH. It's softer yes, but Sodium is not a needed plant macro like Calcium and Magnesium, I would strongly advise against water softener treated water for aquarium use.
 

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If you're referring to water softener, DI or de-ionized, it's not a great idea as you're replacing Calcium ions with Sodium, which tends to shift the water to an even more alkaline extreme of pH. It's softer yes, but Sodium is not a needed plant macro like Calcium and Magnesium, I would strongly advise against water softener treated water for aquarium use.
Water softeners and deionizers are different things. They both use ion exchange concepts. Water softeners replace all cations with either sodium or potassium and all anions with chloride. A deionization systems charges the ion exchange resins with acids and bases. It replaces all the cations with H+ and all the anions with OH-, effectively turning all ionic species into water. None of the drinking water systems would be water softeners, but ion exchange systems are often less expensive to run that distillation or RO systems, and can be run at much higher flow rates. They dont get the water as clean as a good RO system, but fine for drinking water. I haven't seen one designed for home use, but they are common commercially.
 

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Or, depending on what you're keeping, just leave it alone. My kh runs about 19, and I keep most fish and plants (low-medium tech only, I don't do CO2) without problems.
 
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