Keep in mind I lowered this salinity over the course of a week and a half but pulling out a gallon of tank water and replacing it with fresh water each day until I got it low enough. I would think 2 gallons evaporated from a 37 would spike the salinity but it would do it gradually and that's wont kill the fish instantly, BUT when you throw 2 gallons of FW in and it mixes and the SG goes from say 1.031 where the evaporation left it to 1.023 ish you'll shock some sensitive fish and/or kill inverts.