Virtually every aquarist has overfilled his or her aquarium with RODI water when doing their routine topping off. Usually, it’s just a gallon or two too much, and it can be remedied by just letting evaporation take its place. But there are those situations where the overfilling is rather epic, maybe 10-20 gallons if your sump can hold it. Heck, you might even fill your sump to the point that it overflows, spilling water into your aquarium stand and all over your equipment. If the excess filling is that significant, the best thing to do is siphon it out and double check your salinity over the course of the next 24 hours or so to make sure it hasn’t dropped too far. If it does drop significantly, it needs to be corrected, but slowly. To accomplish this, just add a little salt to your top off water, and keep doing so until your tank is back to normal. If the salinity has dropped a significant amount, you might need to take more drastic corrective actions, such as a large water change…but again, do so with only slightly saltier water. Do not, by any means, try to bring that water immediately back up to the proper salinity. The back and forth swing could cause a few issues.