Some freshwater kits give false readings when testing saltwater. I belive the kits are not calibrated for the salinity level of the water or quite possibly other trace elements that are in the saltwater react with undesirable results with the FW
If Aquarium Pharm kits are like this I cant tell you but I do belive I remember trying a freshwater aquarium pharm kit on some saltwater when I first got into salt and it would give me a positive reading for ammonia the instant I tested fresh made saltwater.
THe link you gave shows the master kit for freshwater use only where as above many of the indiviudal kits say FW
use. On another page the Hagen mini master kit reads.
pH (6.0 - 7.6) - (freshwater only)
High range pH (7.4 - 8.6)
Ammonia - (freshwater only)
( Carbonate Hardness)
While it costs a bit more you should make sure your kits say they will work with SW
. Else you could get false positives or worse false negitives.