I’ve always run a pure RODI top off sump under my tank, and never had problems. I guess on this particular day something inspired the water to evaporate at an unusually high rate – causing the top off to push extra fresh water into the tank and in turn cause the salinity to crash. Several pieces were on the verge of having to be thrown away (including my best piece of Palmer’s Blue Milli)

I quickly set up a station of 5 gallon buckets and began mixing more salt water. Pulling a gallon out of the tank, mixing with higher concentration solution in the bucket, then topping off the tank – checking the SG and repeating. After a couple hours of this exercise, I got the salinity back up to 1.025 and called it a night. The levels appeared stable over night, and this morning I did another water change leaving the salinity safely at 1.026. I’ve detached the top off for the day, and will see how much evaporates throughout the day today.

Moving forward, I guess I’m going to have to add a little salt to my top off solution. As it turns out I’m usually low on CA and MG anyway, so making a batch of reef crystals salt water with a slightly diluted salinity (1.020?) might help keep it more stable. This whole process has been very frustrating to say the least, and now I’m even considering upgrading my control to allow a Specific Gravity probe to help keep this in check. For now, daily manual checks will have to do.