15% of how many gallon tank? I would go ahead and be mixing more SW
up for additional PWCs over the next few days. It wouldn't hurt to have extra SW
on hand for a little while just to be safe. Make sure that you get the water up to the temp of the main tank before doing your PWC
...It's also a good idea to allow it to mix with a PH for 24 hours before use. I know sometimes you have emergencies like this and have to do what you have to do. I would just try and get it to temp and SG
at the very least.