This study was done to determine if fortification of a commercial aquafeed with KCl could improve the survival of juvenile Australian snapper Pagrus auratus reared in K+ deficient saline groundwater (KDSGW; <5 mg K+ L−1). Experiment 1 (Exp. 1) tested whether feeding an aquafeed fortified with zero, 25, or 50 g KCl kg−1 for 6 days affected feed intake and survival of fish transferred immediately from estuarine water to KDSGW of the equivalent salinity (20 g·L−1). Experiment 2 (Exp. 2) investigated whether an aquafeed fortified with zero, 10, or 25 g KCl kg−1 affected survival, feed intake, and growth rate (SGR) of snapper reared in KDSGW fortified to have 40% or 100% the [K+] of equivalent salinity estuarine water (20 g·L−1). The results of Exp. 1 demonstrated there was no benefit of fortifying aquafeed with KCl; fish transferred into KDSGW stopped feeding and developed symptoms akin to tetany. Some individuals also died and others became moribund. Exp. 1 was terminated according to animal care and ethics guidelines. The results of Exp. 2 indicated the amount of KCl added to the aquafeed did not affect survival, feed intake, or food conversion ratio (FCR) of snapper, irrespective of water treatment. However, SGR and FCR was better when fish were reared in normal estuarine water and KDSGW fortified to have 100% the [K+] of equivalent salinity estuarine water. Our results demonstrated that juvenile snapper were unable to utilize the KCl added to the aquafeed and were probably reliant on sequestering K+ ions from the water column in order to maintain functions involving hydromineral homeostasis. Fortification of aquafeeds with KCl does not ameliorate the negative effects of KDSWG on the survival of juvenile snapper.