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GH/KH screw-up: 35-40° GH (hardness) and 30° KH (buffer). Going to slowly do water changes to bring it down.


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My tap water is 17° GH and 8° KH. I had a 25g aquarium sitting for about 10 months with live plants (rotala, anubias, ludwigia, elodea, moss etc.) and some snails. In the recent months I did very few water changes. Before getting the liquid API GH/KH test, I had test strips which showed values at the top of the range (180ppm GH/240ppm KH) which was close my tap, so I thought things were normal.

Other values were fine. Ammonia zero, nitrite zero, and nitrate close to zero. I was barely putting any food into the tank. 8.2 pH. 

Got 5 Oto cats two weeks ago, drip acclimated them. Frequent testing with the API Master test kit, everything that I could test for was still fine (zeros for ammonia, nitrite, nitrate, 8.2 for pH). I didn't do big or frequent water changes - only about 10 percent weekly. Otos are eating blanched zucchini hanging from a skewer.

Fast forward to today. Got the test kit. 35-40° GH and 30° KH (I got lazy with shaking the test tube at every single drop for the GH test). I'm somewhat surprised my Otos didn't get shocked when I put them in my tank, or that they aren't dead yet. Going to do daily 10% water changes to bring down the hardness without shocking them too much with a parameter change.

Scape: Sand-capped organic soil (Green Diamond blasting sand on Root Farm organic hydroponic potting soil). Hardscape is aspen poplar driftwood. I've added some aspen and oak leaves. No rocks. 

Filter is a USB Nano air-powered Mattenfilter with a pre-filter sponge.

Any theories on how the hardness is so high? I'm fairly sure there's nothing leaching minerals in my tank apart from maybe the shells of dead snails. I also wasn't doing just top-offs during the ten months, although maybe evaporation could have concentrated the water.

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Do you have a LFS that can back up your test results? My guess is evaporation but there is no reason it should be that high. Especially since the driftwood would be leaching tannic acid which should be forming carbonic acid with your KH and lowering it. That and leaves decomposing should also further acidify your water.


The snails pull calcium carbonate from the water to form their shells so I don't think their death would be able to cause a noticeable shift upwards.

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I'm fairly sure I'm testing correctly, just because my tap water tests is close to matching the water reports from my city (213 ppm CaCO3 last year October, I'm not great at figuring out which conversion to use). The KH/GH tests are also pretty hard to screw up. I will, however, test again and maybe take a trip to my LFS. 

Update: After two 10% water changes, GH at 25° and KH at 22°. (Took the sample right after the water change - water may not have mixed completely together, so I may have gotten more of the tap sample.) Maybe I screwed up the first tests a little with inconsistent drop sizes, particularly the GH test. However, this is still some incredibly hard water.

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