It’s weird but after you use ro/di for so long my tank actually lets me know when the filters need changed. I know if I see any sign of cyano then I need to change my filters.
What do you have for lighting and what have you tried to keep? I started with stylophora first before moving to Acros. This was also a tip from my LFS that if I could keep my stylos thriving then I could move to harder SPS.
My local water is only 50 or so TDS and I'd still be terrified of keeping SPS in it. Heck, I have the best equipment possible and I'm still struggling with these stupid sticks.