Yes, you can put two filters in a fish tank, and in many ways, this can be very beneficial to the creatures in your tank. However, there are also a number of considerations you need to keep in mind before doing it. It is not as simple as just adding a second filter and having an even cleaner tank. In fact, a second filter can prove to be quite complicated.
Having a second filter is often known as redundant filtration. The idea is that there is a second filter that does not really do any of the work. However, it is there as a backup. By this, we mean that this second filter will kick in if your main filter was to fail or break.
This method is often used in more established tanks. It can offer you peace of mind, especially if your tank is well established with a good bacteria colony. Having a spare one will make sure all is not lost, as both of the filtration systems will be able to establish their own bacteria colony, meaning you always have one to fall back on should the other go bust.
This is especially handy in situations where you may need one new filter. Introducing a new filter into an already established tank can be tricky and time-consuming. If you already have a working spare that is used to the conditions of the tank, you can just rely on this until the new one is established. This also poses less of a risk to the delicate conditions in your tank.
However, there are also downsides that you need to consider, too. Bear in mind that new tanks may just need one filter, to begin with, as you will need to build up the bacteria colonies and establish the pH levels within your tank first.
You may also want to bear in mind the fact that it is often the bigger tanks that will benefit from two filters. Smaller tanks may struggle with two filters thanks to the increased water flow created. This can have a knock-on effect on your tank life who may not be able to cope with these new conditions.
Another thing to keep in mind is that having two filters actually increases your workload when it comes to cleaning and maintaining them. Sure, you may have to clean them less, as two of them will be doing the job, but the fact still stands that when you do need to clean them you will have to do the process twice over. You will also still need to clean your tank regularly, as well as keeping up with water changes. Just because you have an extra filter, does not mean you can get away with less cleaning.