Salt is the compound produced by combining an acid with a base. Common household salt, including table, rock and sea salt, is the combination of sodium hydroxide, the base, and hydrochloric acid, the acid. According to the Aquascience Research Group, all forms of household salt are basically the same, but there are applications for which one salt is better suited than another. Rock salt can be used anywhere that pond salt can be used, but the reverse is not true.
Rock salt, a chunky substance, comes from underground mines. It's in its natural state and hasn't had impurities removed. Pond salt is a finer grade of sea salt. Its target use, as its name suggests, is in fish ponds.
Ponds and Aquariums
According to the Aquascience Research Group, salt can be a useful additive to ponds and aquariums for the treatment of nitrite poisoning and osmoregulatory stress, the latter being unlikely in a freshwater environment. According to the Encyclopaedia Britannica, osmoregulation is the "maintenance by an organism of an internal balance between water and dissolved materials regardless of environmental conditions." Osmoregulatory stress occurs when a fish is unable to maintain this internal balance and its system is taxed by a change in the environment. Additional salt can help rebalance.
It is necessary to remember, however, that the addition of salt to a freshwater environment can kill plants and some sensitive fish.
Many pond supply stores recommend the use of pond salt for maintaining healthy ponds, suggesting that the salt protects the fish from parasites and restores electrolyte levels for healthy gill function. However, Aquaworld Aquarium recommends the use of a salt bath outside the pond or aquarium to address external parasites on fish. Salt can also be used in a quarantine tank to help injured fish with osmoregulation while healing.
Either rock salt or pond salt will work in ponds and aquariums, but pond salt will dissolve more quickly.
Neither pond salt nor rock salt should be consumed as each substance contains impurities.
Because of its larger size, rock salt is often used in cooking to make salt-crusted dishes. The food is encased in a salt crust, sealing in moisture and baked. Before the food is consumed, the salt crust is removed and discarded. A finer salt size, such as pond or table salt, doesn't work because it simply melts rather than forming a crystallised crust.
Ice Cream Making
Rock salt is frequently used in making ice cream. The salt does not come into contact with the ice cream itself, but is used to lower the freezing point of the ice and speed its melting, which, in turn, freezes the ice cream. A finer salt, such as pond salt, can also be used in many ice cream makers, but it takes longer for fine salt to melt the ice because the salt dissolves rapidly and requires frequent replenishing. A larger quantity of fine salt is required, resulting in an increased cost.
Rock salt is often used to melt ice on sideways, driveways and roads. Some rock salt sold specifically for this purpose has also had chemicals added to further speed the melting process. Pond salt's smaller crystal size makes it impractical for this purpose.
And the Winner is ...
Pond salt has limited usability as an additive for ponds and aquariums and can only be bought through pond and aquarium supply stores. It also tends to be a little more expensive than rock salt because it's considered a speciality item. Rock salt can also be used in ponds and aquariums, but can also be used in other ways. It is more readily available in grocery stores, hardware stores and mass retailers.