Taking the mayhem out of marine fish keeping
EQUIPMENT FOR YOUR MARINE TANK
Many people come to the marine fish keeping hobby from a fresh water tropical fish background and are, therefore, well familiar with the idea of using external canister filters as their main form of filtration.
Your marine tank filtration is different to fresh water as the bacteria you will rely on for filtration lives within the tank itself rather than on the sponges and media associated with external canister filters. As you start doing further research into keeping marine fish you will start to read many snippits of ''don't use canister filters' and 'canister filters are nitrate factories'. Whilst I agree that if not used correctly, canister filters can lead to long term nitrate problems I certainly don't agree with idea that you should never use them.
In an ideal world you wouldn't have the need for a canister filter. Indeed I have just spent a whole page describing how filtration is carried out within the tank itself, by all the different types of baceria that will set up residence on your live rock aquascape. The rock provides a far more superior envronment for these bacteria than a few sponges ever could so why then, would you even consider a canister filter?
The simple answer is that if you are fortunate enough to be able to have a large tank, great skimmer and a decent sized sump then you would have no need for an external canister filter. Life, however, isn't always perfect and you won't always have these luxuries. You may have a small tank, no room (or money) for a protein skimmer and no sump. This is where canister filters become actually quite useful. Think of them as mini sumps, if you are, for example only able to run a 30 or 50 litre tank then a 6 litre canister filter can add a significant volume of water to your overall system. Maybe you have a 200 litre system but either no protien skimmer or a skimmer that doesn't have a chamber for extra media. In these cases an external canister filter can be a great addition.
Don't be afraid to use the one you have lying around, left over from your fresh water days, a good rinse out with tap water will soon get it back to a suitable condition for use in your marine tank. The main difference when using it for salt water is to remember you are not using it for filtration, its sole purpose is to provide a place for you to put stuff. With this in mind get rid of any sponges, hoops or anything else that may be in it and get it back to an empty state. They usually come with a few plastic trays where in the fresh water world you would put nitrate reducing filter media.
Once your filter is back to its birthday suit you are able to fill the trays with any of the media you wish to use. The most common being a phosphate reducing media. Some people put small bits of live rock in them thinking it will add to the 'live rock' filtration. Don't, all that will happen is the live rock won't get sufficient flow around it to work efficiently and will trap detritus, this, in turn, will just raise your nitrate levels. Just use the whole filter as a container for your chemical filtration. Some people run carbon in the external filters. I would suggest you only run carbon now and again as it can do more harm than good by attempting to strip out every organic in your water. Although you certainly want low levels of organis in your marine tank you don't want the whole thing becomming too sterile.
Don't be put off by the thousands of people that tell you it's a bad move to run a canister filter. They can positively contribute to your marine fish tank if used correctly.
If you are running a very small tank (known as a pico) then consider the benefits of the additional water volume.
Filters - in a nutshell
If used correctly an external filter can be a great addition to your marine tank
Make sure you maintain you canister filter correctly otherwise you will end up with nitrate problems down the line
Remember your main source of filtratioin will always come from your live rock, don't think of it as part of your filtration system, rather somewhere just to put chemical filter media
Don't stress too much over changing your external filter over to marine. After a good wash in tap water it will be ready to go
Don't be tempted to put live rock in your canister filter, although it is the media your bacteria will live on canister filters don't offer the right conditions for the bacteria to thrive so you pieces of live rock will just end up collecting detritus and causing elevated nitrates
If you are considering running carbon don't run it for too long as it can make your water over sterile which won't necessarily do your tank inhabitants any favours