What's really an acceptable GH/KH level? - The Planted Tank Forum
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post #1 of 3 (permalink) Old 06-21-2015, 05:02 AM Thread Starter
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What's really an acceptable GH/KH level?

I'm sure you guys get this alot, but after doing my research, I still don't know what I need to have these levels at.

I am nearly ready to flood my HC carpet, and I'm going to stock cardinal tetras. According to the API test kit, it says plants and tetras should be between 0-3 dGH, and i've read on the forum that HC will tolerate any GH level up to 15 or so.

My tap water GH is 11 and KH is around 6.

Also, i've been needing a lot of advice and have been making many threads lately. Nobody at my lfs knows planted tanks like you guys. Thanks for your responses everyone.

Last edited by dasit88; 06-21-2015 at 05:05 AM.
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post #2 of 3 (permalink) Old 06-21-2015, 03:56 PM
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Some fish and some plants will do better in hard/soft water, but generally they all do fine in a wide range of both GH and KH. Your local fish store probably keeps their fish in tap water, so you know those fish are acclimated to your tap water.

One possible problem with GH is that if yours is from calcium ions only, with little or no magnesium, you do need to dose magnesium. Most of us, I think, use a GH builder, which contains both calcium and magnesium (and potassium, usually) at water changes to be sure we have adequate magnesium.

Low KH is very good for many plants, and doesn't harm the fish. But, high KH is tolerated by fish when they are acclimated to it. Low KH would be 1-3 dKH or so, and high would be above 6-8 or so. I use water that mostly comes from the local river, and it is soft, low in both GH and KH. I don't bother to measure either one, and I ignore KH entirely.

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post #3 of 3 (permalink) Old 06-22-2015, 02:02 AM
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There's no simple answer to what GH is appropriate for plants or fish. When you hear "this fish likes soft water" they are referring to low GH not PH as most assume.

Plants are a different story. Ninety five percent of the plants we keep can be kept at widely different GH and KH levels.

When it comes to plants forget the GH number and concentrate more on each nutrient. Calcium and Magnesium are the two major constituents that make up GH. The problem with using GH only is that it's a total measure.

The major players are Calcium and Magnesium. However, Sodium, Potassium, iron and Manganese can contribute to total GH. So what magic GH number is needed? Impossible to say.

A calcium level of 15 ppm and Mg of 5 ppm would be ideal for most plants. That's about 3 degrees GH. Reviewing water reports can yield this information.

There is a formula you can use to determine GH from each nutrient. First calculate the Calcium carbonate equivalent using this formula when each nutrient is known (water report or dosing).

CaCO3 = 2.5 * Ca + 4.10 * Mg + 2.18 * Na + 1.28 * K + 1.79 * Fe + 1.82 * Mn

Then to get GH divide CaCO3 equivalent by 17.86

Is it harmful to add the above amounts to make sure we always have enough? Not at all. This is one of the premises of the EI method. It may not always be needed but it generally won't hurt a thing so simply add the extra when you don't know the individual breakdowns.
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