If you’ve mixed up a container of new saltwater but haven’t gotten around to actually performing the water change for a week or two, be sure to test the water before using it. As the water sits and mixes for a long period of time, the alkalinity and pH will decrease the salinity may climb as some water evaporates. On top of that, the longer the water is mixing, the more time it has to be exposed to contaminants. Because the parameters can get so far off course, using the water to perform the big change might cause more harm than good. Because of this, we recommending checking the water parameters on newly mixed saltwater that sits out longer than a few days. If the parameters get out of line, you could always try dosing various additives to get them back to the proper levels.