Definitely get on those cartridges. The sediment and carbon filters need to be replaced every 6 months. As for the membrane and DI
resin, you really need a TDS
meter to determine when those are going. I got my meter on eBay for less than $40.
Like FF said, you could test your IO. I believe they claim there is no PO4
in the mix, as most salt manufactures say this.
I love my Oceanic salt mix. The CA
level is super high. It is around 480ppm CA
when mixed to a salinity of 35ppt. The Alk
is a little lower than I would like, but that can easily be increased in the mixing container.
The reason I changed to Oceanic was because I hated the chronic levels of low Ca
in the IO. It never tested over 350ppm of Ca
for me when mixed at a salinity of 35ppt.