First of all, the important thing to note is that arguably the most important role of a filter is to house all the important bacteria that helps cycle your aquarium. There are many different types of filters on the market. You have canister (or external), internal, hang on the back (HOB), sponge filters and also under gravel filters. Under gravel filters are becoing less popular on the market, but they are an excellent additional filter! Whenever you're looking at filtration, it's important to note that most brands are quite generous when advertising the filtration amount of each filter. At HFK, we would say it's a good rule of thumb to try and get a filter with a filtration amount that's double the size of your tank. For example, we would put a filter that advertises it can filtrate 110 litres in a 50-60 litre tank. If you have a 100 litre tank, it's advisable to get a filter that states it can filtrate 200 litres or above.
When thinking about whether or not you need to replace your filter media, you might read up about whatever filter you have, and chances are that the brands say to replace the sponges every month. We do not recommend that you do this. Changing your filter media can, and will, crash your cycle as you're removing all the bacteria that you've worked hard to cultivate! The media in filters can sometimes last years. Whenever you're doing maintenance on your filters, check the sponges and check everything else. If they look good, undamaged, and just a little dirty, give them a squeeze in your bucket of tank water whenever you're doing a water change. This is the best method when it comes to keeping your bacteria where we want it, which is in the filters. If your sponges are past their best days, then replace the media small amounts at a time. This gives the bacteria a chance to grow on the new media you're putting in, and you're less likely to crash your nitrogen cycle. The only exception to this is Carbon Filter media. This is due to the fact that carbon filters tend to "hang on" to different chemicals in your water, and when it can no longer hold on to any more chemicals, the carbon media will no longer work as intended.
On to filter types, we'll go through 4 different popular types of filter which will help you decide what is best.
#1: Internal Filters
Internal filters are, as the name suggests, a filter that is placed inside your aquarium. These are particularly useful if you don't have a lot of room around your aquarium, as it goes inside the tank instead of outside. Normally, these are better off in smaller aquariums or more lightly stocked aquariums as they don't have as much of a filtration capacity, but they can be very compact which makes it easy for them to be tucked away into a corner of the tank. They can also be used to agitate the water surface, which will help oxygenate the water for your fish. These filters don't hold as much filtration media as an external filter, which may lower the amount of fish you can have in a tank or restrict what size of tank you can get, but they're extremely easy to set up and use. You just attatch it to the tank, ensure the water level is right and you're good to go! If you're going for a cleaner looking style to your tank, then this filter can be quite a large obstruction to work around, and it can be dificult to try and hide behind plants. If you're just looking for something that can house bacteria in a smaller or lightly stocked tank, then this option is generally cheaper and available in most stores.
#2: External Filters
External filters sit outside the aquarium and water is pumped through it by hoses, and then returned to the aquarium by a seperate hose, which normally has a spray bar fitted. You'll only see the intake/outtake pipes within your aquarium itself. This is a much more powerful filter and more popular in tanks larger than 100L. External filters have much more room for placing filtration media, and as such has a very large surface area for the good bacteria to grow on. Some brands of external filters come with a slot to place a heater, and some also come with a "pre filter" which you can easily take out and clean, and this catches all the detritus before it reaches the other filter media. This makes it easy to clean and keep water flow high. External filters require a bit more space than internal or HOB filters, as they need to be placed either in the tank stand, or beside it. If clean-looking tanks and lots of filtration is what you need, then this one is your best choice.
#3: Hang on Back (HOB) Filters
Hang on back filters are perfect for your smaller aquariums, very accessible for maintenance and are easy to install. Some brands of HOB filters "self prime" which means that you can just turn them on and they'll automatically create a waterflow. Hang on back filters help if your aquarium space is limited, as the filtration is all outside the tank, which leaves you more room for aquascaping/decorating. HOB filters are better suited to smaller aquariums, but have excellent space for mechanical, biological and chemical filtration. Most brands have an adjustable flow, so if you have a long-finned fancy fish (such as a betta) then you can adjust the flow to prevent the fish from getting caught in the stream of filtered water as it goes into the aquarium.
#4: Sponge Filters
Sponge Filters work by using compressed air into your aquarium creating a small vacuum. Drawing in the water through the surrounding sponge, the sponge then creates a breeding area for beneficial bacteria much like the other 3 filters. There's no real limit to how much water sponge filters can clean due to their simplicity, you only require an air compressor commonly found in the aquarium world to suit your needs. A major upside is they're the safest filter for baby shrimp with no way to get caught inside. A negative, at least for some is that to do maintenance on the sponge, once the sponge is disturbed a lot of the detritus will be released into your water column creating a cloudy aquarium for a hour or so.
The take away from filters in our opinion is that it really comes down to what you want from a filter? The safety and simplicity of a sponge filter, the filtration capacity of a canister filter or a smaller internal filter really comes down to what you're stocking and looking to do with your aquarium in the future. There is no wrong answer as long as your filter can meet the capacity of your aquarium.
Our next Fishkeeping Focus will be all about how to put your fish safely into their new aquarium, and we aim to make it as stress-free for both owners and fish as we possibly can.