Have you been using it regularly through that? I know the API nitrate test tends to separate in the bottle over shorter times than that. Shaking the bottle is enough to keep it going through regular testing, but 20 years... who knows, particularly if it sat for much of that time. I think I'd go out and replace it just in case - you can compare its results to a new test kit and see if any of it is still accurate if you want.
