The plumbing doesn't break down but the spa is 20 years old and it might be time for a replacement soon. You probably have some Bio-Film build up in the plumbing which is putting a demand on your sanitizer levels. I would use a purge product like Ahh-Some to do a deep clean on your plumbing. Ahh-Some can be purchased at some spa stores, but you may need to find it on Amazon.