Thanks for all your replies, I appreciate it!
@DaveK I already tested my RO/DI water. It was showing 0 ppm and 0 nitrates, phosphates and nitrites.
I may have those pollutants in my main tank but like you said, it's showing almost 0 because algae consume them.
Before buying a pouch of ChemiPure Blue I researched a lot on the web. I remember a lot of people stating it was a mix of GFO and ChemiPure Elite (this one being a mix of Carbon and Phosphate remover). Are they wrong?
@Therapy All my fish, snails and corals are doing well now; no one's missing.
Because of this algae problem I started to slow down the feeding. Fish eat every pellet I throw in the aquarium. I feed them every 3 days. The anemone is fed with mysis shrimps in small quantities, one time a week.
My RO/DI unit is about 7-month-old now. It's coming from AquaSafe Canada (never heard it wasn't good for aquariums). Could my resin or filter be "malfunctioning"?
Would it be worthwhile to try dechlorinated tap water for my next water change?
@chipmunkofdoom2 I didn't realise Alk had a direct effect on PH but now you explained it, that seems obvious! I will buy a test kit for Alk, Ca and Mg to be certain and post my results ASAP.
@Oxylebius I heard a lot of theories about chloramines. My city is not even sure about the levels of chloramines in our water as it can very from day to day.
I'm not sure it's really the cause of my algae problem, though. If my tank was full of chloramines, I presume fish and corals would show, at least, some signs of discomfort.
Thanks again.