Cleaning a hot tub filter using the soaking method is a great way to get the pleats in the cartridge filter looking clean again. There are many commercially sold overnight filter cleaners on the market, many of which are based in Muriatic acid.
Get an overnight filter cleaning solution bottle here.
Gloves should be worn and care should be taken whenever working with any acid liquid. Typically the muriatic acid is diluted into a big bucket of water and the filter is then soaked in it overnight. After the soaking, the filter is rinsed off with a garden hose prior to use.
Typically, the filter should be washed off every week or so depending on use, and it should be placed in an overnight filter cleaner about once a month for a thorough cleaning of the pleats. After a year, the filter cartridge can be replaced.
To clean the filter overnight, simply locate the filter inside the filter canister (assuming the spa has a cartridge type filter and not a D.E. or sand filter). If the filter is cracked or in non-serviceable condition, a replacement should be ordered instead. If the filter is just dirty, the use a garden hose to get most of the dirt and debris off of the filter. Do not use a pressure washer as that can damage the pleats. There are hose attachments such as filter wands that can make the spraying of filters an easier task, otherwise use a hose nozzle or get the thumb on the end of the hose and squirt away.
It is recommended to use gloves and goggles when handling acid. Follow the directions on any commercial cartridge filter cleaner. If using muriatic acid, then use 1 part acid to about 20 parts water and make sure there is no acid fizzing or bubbling prior to use, or dilute further. Be sure not to get acid on clothing or skin. Muriatic acid also should only be used in well ventilated areas and kept away from kids and pets. Store in a dry, safe place when not in use and be sure the container is properly labelled as dangerous.