Prior to water changes. IMO
If your levels are stable you don't need to do a water change. My reef has been without water changes for 4 months now. Everything's at 0, expect nitrates at 2. I have several clams and anemones. No sps
I test and change my water every week. Even if the parameters are ok, changing the water replenishes the trace minerals. I also clean my sponge filters in the changed water.
Sent Via the R2R Forum APP
12g Nanoreef. Zooanthids, Ricordia, Star Polyps, Acans and two clownfish. CF Lighting, 75% actinic blue, 25% 10,000k white.