When I was working with my spa recently I realized the spa at the house that I have had for a year has an ozonator that was hidden under the tub. Since the tub was installed by the previous owner, I didn't know what was included with it.

Are there parts to the ozonator that should be be replaced after awhile? I noticed that there is something that looks like a cylindrical cartridge on the ozonator. Should (or even can) that be replaced?

In many cases, some ozonators will have replaceable bulbs for a nominal charge, but unfortunately, it's usually within $30 of a complete ozonator, so you're better off replacing the entire thing.

The only exception to the rule are the newer CD type (Corona Discharge) ozonators that don't use a bulb, but rather use a small CD generator 'chip', which are quite reasonable in price, relatively speaking, to replace.