Many people who have a passion for keeping fish are not able to afford the rather large fees associated with purchasing a brand new outer filtration system. Fortunately, there are many people who want to sell their used systems. There are just a few things that you need to keep in mind when you opt for this route.
The first thing you should do when you enter the used parts market is to make sure that you do not pay for anything before you have actually seen it. This way if you find that the quality of the item is less than desirable when you see it you can still get out of the sale. You should also 
ensure that the filtration system is completely watertight, there are no visible signs of limescale, and that it has not undergone any radical repairs. Many tank owners will use tape and superglue to put them back together. Sadly, these types of repairs are rarely effective.
Once the used filtration system you are interested in has passed all of these tests you will be able to bring it home. Enjoy the installation process and rest assured that your pets will reap all of the benefits.