I have inline meters on my RO filter. I will occassionally pin point check the holding tank to see how the tds creep is going. Once my Carbon blocks are over a few months old then I will do a Chlorine/chloramine test on my holding tank when I fill it. I think it's called a free ammonia test but i'm not sure. I've tested my holding tank for Phosphates a few times over the years and it always reads undetectable on Salifert kit. My RO/DI storage is a 30 gallon brute and It's lucky if it gets hosed and wiped out once per year. It usually has a lid on it.