Topping up a softener is a two minute job with a couple of details that make the difference between a tank that works and one that quietly stops drawing brine. The main one is counterintuitive: filling the tank as full as it will go makes a salt bridge more likely, and a salt bridge is the single most common reason a softener stops softening while appearing to have plenty of salt.
Check before you pour
Push a broom handle or a stick straight down through the salt in several places. If it goes through freely to the bottom, the salt is loose and you can top up. If it meets a hard crust with a space beneath, that is a salt bridge, and adding more salt on top of it makes the problem worse and heavier to clear. Break the crust first, scoop out the loose debris, and only then refill. Doing this check every time takes seconds and prevents the fault entirely.
How much to add
Fill to no more than about two thirds and leave a clear hand's width of space at the top. A tank filled to the brim is more likely to bridge, because the weight compresses the salt and humidity does the rest. Keep the level above about a third, since letting it run right down means the brine is not saturated and regeneration is incomplete. Between those two marks is where the tank should live, and marking the level with a marker makes the interval obvious.
Which salt
Clean pellets dissolve with little residue and are the straightforward choice. Rock salt is cheaper and carries insoluble grit that settles in the bottom of the tank and eventually reaches the injector screen, which is where most softener blockages begin. Potassium chloride can be used instead of sodium chloride, generally consumes more per regeneration and costs more per bag. Do not mix types in one tank if the manufacturer advises against it, and store bags somewhere dry, because damp salt cakes.
Use the level as a diagnostic
Mark the level and note the date when you top up. A level that stops falling means the softener has stopped drawing brine, and the causes are a bridge, a blocked injector, a stuck valve or a restricted drain. A level falling much faster than usual means something has changed: a leak on the softened line, a valve regenerating too often, or higher water use. The tank reports faults before the water quality does. Our salt delivery guide covers standing orders.
Questions people ask about adding salt to water softener
How full should the salt tank be?
No more than about two thirds, and above about a third. Filling to the brim makes a salt bridge more likely.
How do I check for a salt bridge?
Push a stick straight down in several places. A hard crust with a space beneath it is a bridge and must be broken before refilling.
Which salt should I use?
Clean pellets. Rock salt carries grit that settles and eventually reaches the injector screen, where blockages start.
What does it mean if the level stops falling?
The softener has stopped drawing brine: a bridge, a blocked injector, a stuck valve or a restricted drain line.