Salt is only used in one part of a household water treatment train: the brine tank of an ion exchange softener, where it regenerates the resin. Nothing else in a filtration system consumes salt. Choosing between pellets, crystals, rock and solar salt is mostly about purity, and purity is mostly about how much maintenance you want.
What the salt is actually doing
A softener works by passing hard water through a bed of resin beads that hold sodium and swap it for the calcium and magnesium that make water hard. Once the beads are loaded with hardness they stop working, and regeneration reverses the swap: a strong brine solution drawn from the salt tank floods the resin, the concentrated sodium displaces the captured hardness, and the whole lot flushes to drain. The salt is a consumable reagent, not a filter medium. That is why the water leaving a softener is not salty: the sodium goes onto the resin, not into the glass, apart from a small exchange.
Pellets, crystals, rock and solar
Evaporated salt pellets are the purest common option and leave the least residue in the tank, which is why most makers recommend them for modern demand-initiated valves. Solar salt crystals are produced by evaporating seawater and are almost as clean, at a lower price. Rock salt is mined and cheapest, but it carries more insoluble mineral matter that settles in the bottom of the brine tank and eventually has to be shoveled out. Potassium chloride is the alternative for households avoiding sodium or discharging where sodium is restricted; it works in the same valve but you typically need to set a higher dose.
Bridging, mushing and the tank routine
Two failures account for most calls about a softener that suddenly stopped working. Bridging is a hard crust of salt spanning the tank with an empty cavity underneath, so the water level never touches the salt and no brine is made: the tank looks full and the water is hard. Break it with a broom handle, carefully, and keep the tank in a dry, temperature-stable place. Mushing is fine salt recrystallizing into a sludge at the bottom that blocks the brine draw, and it is more common with rock salt and with tanks kept overfull. Keep the tank about half to two-thirds full rather than brim-loaded.
How much, how often, and what not to add
Consumption depends on hardness, household water use and how the valve is programmed, not on tank size, so watch the level monthly rather than following a rule. Add salt when the level drops to roughly a third of the tank, and let the old salt go down before topping up so it does not compact. Do not mix pellet and rock salt, and do not switch between sodium and potassium without adjusting the salt dose setting. Resin cleaners are worth using on well water with iron, because iron fouling is the other reason a softener quietly fails, as our softener maintenance guide explains.
Questions people ask about salt for water filtration system
Does the salt end up in my drinking water?
Only a small amount of sodium is exchanged into the water in place of hardness, and the brine itself goes to drain. Households on restricted sodium usually add a reverse osmosis unit at the kitchen tap.
Which salt is best for a softener?
Evaporated pellets are the cleanest and the usual recommendation, solar crystals are a good value compromise, and rock salt is cheapest but leaves insoluble residue you will eventually have to clear out.
Why is my water hard when the tank is full of salt?
That is the classic sign of a salt bridge: a crust spanning the tank with a void beneath it, so the water never reaches the salt and no brine is made. Break the crust and check again.
Can I use potassium chloride instead?
Yes, in the same valve, and it suits households avoiding sodium or facing discharge restrictions. It is less efficient, so the salt dose setting usually needs increasing.
How full should the brine tank be?
Around half to two-thirds is ideal. Brim-loading encourages bridging and mushing, and letting it run empty means a regeneration cycle passes with no brine and hard water gets through.