however, without a temp compensating refractometer, you need to have the water at temp to get a good reading. all salt mixes have a clear breakdown of what salt to water ratio yields what results. you have to kind of tinker with it at first since you may not be keeping the tank at the same temp a they used to get their readings (should also be in the salt info). RS pro and RS Blue are different in the specs they yield. i use my little mixing containers and when i fill to the same level to mix salt, i know it takes 250ml of salt to get the right salinity. i don't even need to heat or measure anymore really because its always the same (i do anyways haha).