I have RedSea Reefer 350 90 gal, Apex, MP40,2 RADION XR30 PRO G5, changed to KESSIL AP9X (2), SKIMMER NOYOS, TRIDENT, DOSER4, REFMAT, and many others, give birth, etc. all bought new, long time ago for 10000, I use APEX only because of TRIDENT, I am very satisfied, but still tests HANNA and salifert every week, if I started first, I would go along the path of buying used ones, and the biggest mistake was a dry stone, torment for a year and a half until the tank worked. If I had carefully read the necessary topics and assemblies, I would have saved more than half. Read a wonderful site more and you will understand what is right for you, just time, time is money, in my case more than half was spent incorrectly. And believe me, my tank was super popped.