Well, I now have both city water and well water available, but for many years we had only the well. Our well was hard, lots of calcium and a little iron, but it was carbonated. The pH out of the tap was well below 7 but as soon as it was aerated or agitated for a while it would jump up to 8.3 or 8.4 as soon as the carbon dioxide bubbled out.
There was and still is no chlorine in the well. The water could be used for water changes and most of the cichlid species got used to 40 to 50% water changes right from the tap adjusted for temperature. To drive off the CO2, I used a long gravel cleaner and a gravel rake on the end to ripple the water before it fell into a tank. I still do. This always worked. Except once.
I had a 75 gallon tank I was raising forty Mylochromis fry in. I was hoping to get them big enough to show some color. These by the way are my all-time favorite snail eating fish. They don't pick out the snail meat like a clown loach. They don't crush the shell like many other cichlids. They don't spit out shell fragments or filter them out through their operculae. They just swallow the snails whole. In the morning I would collect a couple double handfuls of snails and dump them into the tank. None of them would hit the tank bottom. Gone! About an hour later I'd try to be around and watch these fish. Sudden puffs of white sand would appear in the tank out of their back ends.
As they grew I didn't spread them out or put them in a larger tank. I just did more frequent water changes until it became 50% once or even twice a day. One day a couple things went wrong with this method at the same time. The first accident caused me to run out of the fish room to take care of it. As I left I must have bumped the fill hose and the gravel cleaner fell off its perch and into the tank. The rippling stopped and the wand filled with water so none of the CO2 was dispersed. By the time I finally got back, the tank had filled, overflowed, and the water was running happily along an apparently well established brooklet to the floor drain. The 5 to 6 inch fish were unconscious and floating on their sides at the top of the tank. Motionless and imperceptibly breathing. They covered the tank surface and were about three deep. I quickly threw in some air stones and sponge filters, whatever was already on the end of the airlines I could toss in that tank. I drained out about half of the water and replaced it with water siphoned from another tank. Before the tank was even refilled, the Mylochromis were swimming around and begging for food like always, as if nothing happened.