If your swamp cooler is looking like it is being taken over by mineral deposits, well…that’s because it probably is. Swamp coolers aren’t something you can just plug in and forget about until the winter. Rather, you will need to do some periodic maintenance.
Hard water certainly isn’t pleasant for swamp coolers. In fact, I know plenty of people who pipe soft water into their cooler. Really though, the cooler works off of evaporation, and what is left in the cooler is lots of salts. You can do several things to keep the environment inside the cooler operating as well as possible.
Most pumps have a bleed function that purges old water from the reservoir and pumps it out of the drain tube to the exterior of the unit. This will keep fresh water entering the unit at a low volume while getting rid of old water high in salts. You can run the tube outside, maybe to a garden, or somewhere you won’t just be wasting the water.
You can also buy water conditioning products which will help to keep the scale low. Most are blocks that you just set in the reservoir and it will slowly erode away.
If your pump does not have a bleed-off function, you can just periodically (every few days) empty the reservoir and refill it.