Hi All; From what I have read, one Reef tank rule or thumb is:- High nitrate, maintain high alkalinity. Low nitrate, maintain low alkalinity. What I have noticed with my tank, since nitrate & phosphate became undetectable using test kits, is that my alkalinity is always low, at around NSW levels. Even after raising it, it drops back down after a few days. This isn't necessarily a problem as I would be aiming for a low alkalinity level, and the pH is fine. But I am curious if anyone else has experienced this same reaction, & what may be the reason? Especially if anyone has had the situation where alkalinity fell too low and it became a problem maintaining it at a reasonable level. I'm guessing that carbonates are stripped from the water and utilised for the production of algae for the same reasons CO2 is necessary. This is why alkalinity remains low. The tank is 50 gal. Live rock, corals, fish - scrubber 4 cube (feed two cubes) - activated carbon and filter sponge 50% of the time only. Water change - 25% NSW every 1 to 2 months. RODI top up.