How Saltwater Aquarium Filtration Works
If you've already decided on what type of aquarium you want to have then it's time to start thinking about and understanding how you're going to filter your water. As you start shopping around, you'll find that there are tons of solid options on the market. If you're just starting to learn about filtration and are perhaps just now getting into the hobby, it's likely that the tank you purchase will come with some type of filter. In that case, it's still really important to understand how that filter works.
Filtration Processes
There are two main processes at play in aquarium filtration:
- Biological filtration is provided by bacteria that's growing all through your tank to convert ammonia (extra food, fish waste, etc) into nitrate.
- Mechanical filtration is provided by some type of system that strains the water and removes larger solids before they have to be processed by the biological filter.
It's important to understand that both processes are required.
Biological Filtration
Your tanks biological filtration is provided by a lot of different strains of bacteria that are growing all over your tank. The point of live rock is to both get this bacteria into your tank and also to provide it somewhere to live. Live rock is very porous which means it provides a ton of surface area for the bacteria to multiply. As the water passes through the rock and over the bacteria, the bacteria grabs the bad stuff in your water (ammonia and nitrite specifically in this case) and converts it to nitrate. Nitrate is far less toxic to your fish. When you change your aquarium water, what you're actually doing is just pulling the nitrate out of the tank.
In addition to the live rock, your substrate (sand or gravel in the bottom of your tank) provides another really good environment for this bacteria to grow. Some filtration systems also include space for the inclusion of "bio-media" which are generally pieces of a porous material like ceramic. This is generally not necessary to use if you have enough live rock and substrate in your tank. It doesn't hurt anything to also use a bio-media, it's just likely not necessary.
What makes biological filtration somewhat confusing is that, unlike mechanical filtration, you can't directly see it. You can indirectly test for it, though, and verify that it's working. When you test your water, you'll test for ammonia, nitrite, and nitrate. The presence of nitrate and the lack of ammonia means that your biological filter is working and that you have a healthy colony of bacteria throughout your tank.
When you first setup a new saltwater tank, you're required to "cycle" the tank. Cycling your tank is just the process through which you actually kick-start the growth of your bacterial filtration and let it build up to the point where it can successfully handle the amount of ammonia in your tank.
The good news is you don't actually need to do anything to maintain your biological filter. The presence of fish, coral, and the introduction of food provides the bacteria with the waste necessary to stay alive.
Mechanical Filtration
The best analogy for mechanical filtration is just a strainer. If you run the water through a strainer, you end up catching the solid stuff in the strainer while the water passes through. This is not always the exact process used in mechanical filtration, but the same basic premise always applies - remove the bad solid stuff from the water. Mechanical filtration is mechanical in the sense that it's mechanically removing waste from your water rather than chemically or biologically.
There are two common ways to do this and, generally, they're used in conjunction with each other:
- Disposable media - commonly filter floss or filter socks - are literally just strainers that catch the solids in the water. They're always at the top of the mechanical filter since they're really good at catching the big stuff before it makes it any farther through your filter. Which of these two you use is going to be determined by how your tank is setup and the actual filtration system that you buy.
- Protein skimmers use really small bubbles to separate any organic matter (things like food and fish waste) from the water. The bubbles move up through a column in the skimmer and pop when they get to the top. The waste then falls down into a cup that you pour out every few days. Protein skimmers are incredibly efficient and effective at removing anything in the water that shouldn't be there.
Chemical Filtration (optional)
Chemical filtration is provided by including granular activated carbon (it's basically just high-end charcoal) somewhere in your filter. The carbon absorbs toxins and other bad chemicals that are dissolved in your water. If you have a functional, healthy biological filter, though, then you won't have high levels of these chemicals in your water to begin with. The carbon may, in some instances, be actually removing too much from the water which will lead to nutrient deficiencies in reef aquariums.
Running Granular Activated Carbon for chemical filtration is really only needed if you've added some kind of copper-based medicine to the tank recently and need to filter it back out or if you have some reason to believe that the water you're putting in your tank includes things like chlorine. In the freshwater world, carbon is also useful for removing tannins from the water that are introduced by including pieces of wood.
