Filters are essential for keeping your water clear and healthy! These days, most of them come with a UVC unit - an intensive Ultra Violet Light that nukes algae as it is pumped through the filter, which stops the water looking like pea soup. They also all have 'filter media' inside to catch and sieve out waste so the water and bottom of the pond stay clean.
Need help choosing? There are 3 mean types of filter. The simplest ones are the Underwater Filters, which are small all-in-one units that sit at the bottom of the pond. They only have one power cable so are the easiest to install, but as they are small filters they are only really for smaller ponds. Pressure Filters look like a little bucket and sit at the side of the pond (but not right next to it if you want to hide it behind a shed or at the top of the rockery). They need connecting to a pump at the bottom of the pond (which will come with the filter if you choose a "set") and are very easy to clean as you just use a back-flush handle on the lid and don't even need to get your hands wet! Gravity Filters are the traditional 'box filters' and are also fed from a pump at the bottom of the pond. The main difference is that they fill up and overflow back into the pond, so they need to be installed right next to and above the pond level. These are big and ugly (call a spade a spade) but they do a really good job of cleaning because they hold so much filter media.
What size? Filter sizing is based on the volume of your pond, and whether you have fish in there. If you don't have fish, the filter will have an easy job of keeping the water clean so you can just choose a filter that matches your pond volume: if you have a 3000 litre pond, you'd choose a 3000 model filter. If you have fish, you'll also have a lot more waste in the pond so you need to pick a filter twice the volume of your pond: if your pond is 3000 litres, you need to buy a 6000 filter. If you have Koi carp, you'll have LOADS of waste so you need and even bigger filter. Ask us if you need working out what size is best for you.