Backyard Aquaponics
A place to discuss aquaponics

YES, bunnings salt is MUCH lower quality. Salt for swimming pools is evaporated (as mined salt has too many minerals in it), 'first grade' salt such as Olssen's and Cheetham's Mermaid brand salt (Sunray salt aka 'the bad stuff' is also produced by Cheethams) is scooped off the top of the salt bed, filtered and ground finely (which helps it dissolve quickly). 'Second grade' salt aka Sunray is the dregs they get off the bottom, and includes 10% 'undissolvable solids', or to put it another way, mud.

Second grade salt also has a much higher level of minerals in such as copper and iron, which is what causes staining.

Now second grade salt can be used IF your pH is 7.0-7.7 (any higher or lower and the minerals are likely to drop out of suspension, causing stains), IF you brush it in very well to make sure it's all dissolved (otherwise it will cause stains) and IF your filtration is good enough to remove the mud (otherwise, yes, stains).

Stains can occur with first grade salt but it is MUCH less likely, hence why decent pool shops don't stock second grade.

With the salt crisis we had here in Queensland at the start of the year, a lot of my customers used second grade salt from bunnings or woolworths/coles as it was all they could get (and even that was very patchy) and about 5 dozen have since had to remove staining.

Unless there is a salt shortage such as this one, don't bother using it, the difference in price isn't worth it.