Obscenely Low KH.

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Registered Member
Nov 29, 2002
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I was out of town for a month and a half, and I just came back this last week. Like usual, I left a good friend in charge of my fish tanks, and like me, she has been keeping fish for sometime now. We've always trusted each other on these matters, but this time there was a bit of a foul up. I have an Amazonian tank set up in a 29 gallon with a school of Von Rio Tetras (H. flammeus), a school of Cardinal Tetras (P. axelrodi), a small group of Green Cories (C. aeneus, I believe), a pair of Otos (O. affinis), and a Clown Pleco (to lazy to look up the scientific name).

Anyway, when I came back I found out that my friend had been mixing the water for my water changes wrong. My tap generally tests out as ph 7.6, GH 15, and KH 9. I buy R/O water at the grocery store and mix it at a 1:1 (I change out three gallons, or roughly 10% a week) ratio and I get a pH of 7.0, a GH of 7, and a KH of 4. Usually I make four gallons to keep the ratio. My friend I guess heard me wrong and he mixed the water at a 3:1 ratio in favor of the R/O water. This lowered my water parameters quite a bit, giving me a pH of 6.8, a GH of 4, and a KH that bottomed out below my measuring limit (below 1). Two of my cories did developed a swim bladder problem, and after one of them died, I cleared it up with the "greens" recommendation. The other returned to normal after a few days of only throwing shelled peas into the tank.

Now, I'm not sure about what caused the illness. Was it the sudden change in water parameters that caused the illness, or is the low KH? I ask because I'm afraid that if I switch back to my normal ratio I will cause illness in the tank again. So far only the Cories have exhibited any kind of odd behavior, and the other fish seem to ahve adjusted with relative ease to the new conditions. Even my live plants seem, as of yet, unaffected, and I think they would exhibit some signs of discomfort after six weeks of water changes at the wrong ratio. So, I'm wondering if I should continue with the 3:1 ratio that all of the fish seem to now have adjusted too, or is that KH to low? Can the above mentioned fish prosper at such a low KH?



happy new year!
May 18, 2002
I'd say go back to your 1:1 ratio to get the KH to what it used to be. A very low KH can cause your pH to be unstable, the probable cause for the recent cory death.

I also would increase the water change ratio from 10% to 20% weekly and vacuum the gravel in case there were a lot of uneaten food and wastes while you were gone.


AC Members
on the other hand...

My own NYC water is soft and gets softer after the plants have sucked out the last of the calcium. The pH drifts between 6.2 and 6.8. Photosynthesis takes up CO2 and raises the pH during lit "daytime" hours. Then at night the pH drifts down. I guess I could have a pH crash if I spilt a lot of flakes in a tank. It hasn't happened though

Java Fern gets some bent midribs, the Ca is so low. But all the carbon is in carbon dioxide; that's the gain for plants. All the softwater fishes you mention should thrive and will spawn for you.

Check out Randy Carey's characin site www.characin.com Hunt down his article at the site "How Low Can You Go?"

Part of what damages fish in pH changes is likely to be the de-ionization of ammonium (NH4) to toxic ammonia (NH3) as the pH rises.

Just thought I'd post, because this is all counter to the general current...