I have an EV 120 Aqua C that I'd run a year. I did the full-up-with-vinegar thing and found colonies of fanworms and a quarter inch of bottom gunk, calcium deposits in many narrow spots: huge amount of gunk turned to goo overnight and skimmer efficiency improved 300%, no kidding. I also discovered and tightened some loose fittings/connectors to eliminate air leakage and that also helped.