All-in-one underwater filters are a great way to keep your water clear without taking up much space or doing anything complicated with wiring and plumbing! They are simple plug-and-play units with one power cable for the pump and the built-in UVC to stop green water. They all have a 2 year guarantee.
Need help choosing? The OASE Filtral range is definitely the best for build-quality and ease of maintenance - you just pop up the blue clips on the housing and you're IN! The PONTEC PondoRell is the winner if you're on a budget, but it's not as easy to pry open.
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.