I need advice about changing the water in my fishtank....?

Check your pH and ammonia. If they are normal, the water is likely safe. With the potassium in the baking powder you might see a bit of an algae bloom as it provides nutrients to the algae.

The only other impact to water quality is it would increase the hardness slightly and may have raised the pH. If the pH is in the safe range for your fish, they should be just fine. You could do a 50% water change for good measure.

Note Baking powder is not baking soda. They both are Sodium bicarbonate, but Baking Powder also has Potassium bitartrate.

I would suggest 25-33% water changes every 36 hours. For a week just to be safe.

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