You need a product called a water conditioner, or dechlorinator. Think of it as the antidote to the poison your municipal water department so generously provides. You just add a few drops to your bucket of tap water, and poof—the chlorine vanishes faster than your motivation to clean the bathroom.
I once forgot to add dechlorinator and watched my betta fish, Sir Floats-a-Lot, sulk at the bottom of his bowl for an entire afternoon. He literally turned his back on me. Never skip this step. Your fish will hold a grudge.
Temperature: The Goldilocks Zone
Fish are not like you and me—they don’t enjoy a lukewarm shower or a sudden ice bath. They are delicate little divas who need the water to be within a specific temperature range. If you pour cold tap water into a warm tank, your fish will look at you like you just dropped them into a polar bear plunge.
Get a thermometer. Aim for about 76–80°F (24–27°C) for most tropical fish. Let your water sit in the bucket for a few hours until it matches the tank’s temperature. Or, use a heater in the bucket. I’ve seen people warm water by holding a plastic bottle under hot tap water, which is the fish equivalent of microwaving a popsicle. Don’t do it.
