Have you tested your water? If so post the results here.
I'm assuming you are talking about your 20 gallon correct?
How many other tank mates and what species are they? Post every detail as it helps narrow things down.
What is the temp of your tank?
Last summer I had a betta that would swim weird because it was too hot in his tank. Swimming weird can also be a sign of swim bladder issues. Are you sure it just started after your water change? Was the water conditioned or taken right out of the tap?
If you answer all of the questions, it'll help cure your fish.