Here's a nice article on strontium by none other than Dr. Randy
http://www.advancedaquarist.com/2003/11/chemistry
Looks like seawater actually has around 8ppm (not 12ppm) so Daniella's strontium is not very low (6ppm) but that 25% can make some big difference I guess. Did you test it after dosing? I looked into aquavitro salinity and H2ocean's salt mix analysis and they seem to guarantee strontium anywhere between 8 to 10ppm.