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Here, is an interesting e-mail question I received today about using well water for water changes.

If I use a garden hose outlet (to bypass the water softener), what kind of hose should I use? Do I have to worry about adding anything to the water from inside the hose, or do I need to get a special hose?? [from Lee in Lynden, WA]

My response:

I wouldn't even think of buying a special hose for changing water; I just use a regular garden hose. However, if water has been sitting in the hose for a long time, it might contain leached plastics. I run out this old water into the sink for a minute before using it to fill the tank.

The bigger problem I see is that well water can accumulate excessive amounts of CO2 gas and can create major problems during water changes. [Explanation: As rain water percolates through the soil, it collects CO2 from the bacterial metabolism of soil organic matter.] This CO2 can kill delicate fish, invertebrates, etc. One of my water changes killed some of my delicate native fish. The tougher "lake fish" (Blue Gill Sunfish) were fine, but the more delicate "river fish" (Shiners) died before my eyes.

It is especially a problem during the winter when the water is cold. (I have never had this problem in the summer.) That's because CO2 gas is about two times more soluble in cold water (~50F) than warm water (~77F). When I do water changes in the winter, I often see a lot of tiny gas bubbles released from the new water as it mixes with the warm water in the tank. I think the bubbles are CO2 gas released as the cold water warms up and its CO2 becomes less soluble.

Therefore, I would be careful doing a "winter water change with well water". If you see any distress in fish, stop running water into the tank immediately. Excess CO2 works fast, and in my experience, kills fish quickly.

This excess CO2 will degas out of water in a few hours. If during the winter, you add the water slowly (or in stages), the CO2 will dilute out enough that it won't create a problem.
 
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