To answer this, it is better to understand how fluorescent tube works first.
The fluorescent tube operates with ballast. There are 2 types of ballast, non-dimmable ballast and dimmable ballast. When you are going to replace your fluorescent tube, and you currently experience dimming light, so just go and find same wattage same size fluorescent tube. You do not need to worry about finding special dimmable fluorescent tube, because you are using dimmable ballast already.
