I didn't know which to vote for as I test, I change on a regular basis or when my tank looks like I need it. I've slacked off on the testing as of late as I've been too busy (or too lazy) but for the most part I change it out every 2 months whether it needs it or not or when my tank has more algae than normal. I've also upped lately as I noticed that my tuxedo urchin had a serious Fabio hairdo of green hair algae. I also test every few weeks with the reading usually being 0.08-0.015ppm. The algae I grow doesn't seem to need much phosphate it would seem and only grows in areas where I can't see it (back glass) and the fish aren't interested in eating it (back glass).

Haven't used GFO for some time now. No difference in test results (crappy? API test kit) whether I used GFO or not, so stopped. Unable to accurately evaluate results, but it always looked like very low or undetectable to my eye, a very very light blue hue which looked to me less colour than the lowest match on the paper scale. Too costly IMO to be using the stuff when I can't confidently test for it and based on what's out there, I'm not prepared to spend megabucks on premium test kits either. I guess I'm willing to live with the results for better or worse. Only so called problem I see is hair algae, which hasn't killed anything in my system yet. Unsightly, a little, but I don't mind manually removing it periodically, mostly during water changes. Who knows whether the algae is due only to phosphates, although my nitrates are also undetectable... hair algae, chaeto, caulerpa doing their job?? Could well be that my LR, all being from previous owners is contributing to the nutrients feeding the algae. No idea whether previous owners used RODI, I doubt it and I only started using RODI two years into setting the tank up. It appears to be getting easier to control the HA as more time using RODI water passes.