Fitting something to a hose to deal with hard water is a request with one real answer and several products that do not provide it. The reason is chemical rather than a matter of quality: hardness is dissolved calcium and magnesium, and dissolved material passes through anything that works by straining or adsorbing.
Why sediment and carbon cartridges do nothing
The USGS describes hardness as dissolved calcium and magnesium. A sediment cartridge stops particles and dissolved minerals are not particles. Carbon adsorbs organic molecules and chlorine and has nothing to hold a calcium ion with. So a hose filter containing either passes the hardness through untouched, the water dries leaving exactly the same deposits, and the household concludes hose filters do not work. They work; they are simply the wrong tool for this particular problem.
Deionizing resin, which does work
A deionizing cartridge strips both positive and negative dissolved ions, producing water with almost nothing left to leave behind, so it dries without spotting. This is what professional detailers and window cleaners use and it genuinely delivers. The catch is running cost: the resin is consumed by the total dissolved content it removes, so on hard water it is used up quickly, and a full car wash on hard water can consume a noticeable fraction of a cartridge.
Making it affordable
Use the deionized water for the final rinse only, after washing with ordinary water, which is what detailers do and which multiplies cartridge life many times over. Take the hose feed from a softened line if the house has one, since the hardness has already been exchanged and the resin has far less to remove. And keep the cartridge for the job that needs it rather than leaving it in line for general watering, which consumes it for no benefit at all.
What the problem usually is
Two different complaints send people here. Spotting on cars and glass, which is the deionizing case above. And scale on outdoor fittings and irrigation emitters, which is the same hardness depositing and is addressed at the house rather than the hose, by a softener or scale control on the supply. Neither is fixed by a hose cartridge. Drying surfaces before the water evaporates costs nothing and works. Our hard water shower filter guide covers the point of use question indoors.
Questions people ask about garden hose hard water filter
Will a hose filter soften water?
No. Hardness is dissolved and passes through sediment and carbon cartridges untouched.
What stops spotting when washing a car?
Deionizing resin, which removes the dissolved minerals so the water dries leaving nothing behind.
Why does the resin run out so fast?
It is consumed by the total dissolved content it removes, so hard water exhausts it quickly.
How can I make it last?
Use it for the final rinse only, and take the feed from a softened line if the house has one.