The Minimal Role of Mechanical Filtration in Marine Aquariums

Mechanical filtration—the physical straining out of suspended particulates from aquarium water—has something of a mixed reputation among marine aquarium hobbyists.

This may seem odd to those with experience in the freshwater side of the hobby, where the use of hang-on-back and canister power filters is quite commonplace. But then, freshwater fishkeepers tend to think of us salties as a rather peculiar bunch in general—something Chris and I probably should have warned you about before we attempted to seduce you into joining the dark . . . er, I mean the saltwater side of the hobby.

But now that you’re one of us and there’s no turning back, let’s explore the role mechanical filtration plays in marine aquariums.

Why the dubious reputation?

The issue with using dedicated mechanical filters on a regular basis in saltwater systems, where exceptional water quality is non-negotiable, is that they tend to do their job of trapping suspended particulate waste too well. All that trapped gunk—which is merely sequestered within the system, not removed from it—tends to decompose rapidly, so if you don’t clean the filter frequently enough (which is very often the case because “out of sight means out of mind”), the dissolved nutrient level in the system will soon begin to climb.

Also, the media used in mechanical filters can easily become biologically active—i.e., colonized with nitrifying bacteria—if not rinsed or replaced often enough. Then, when you finally get around to cleaning or replacing the medium, you’ll essentially lose a portion of your biofilter.

Many reefkeepers tend to shy away from mechanical filtration not just for these reasons, but also because mechanical filters can trap good particulates along with the bad—including planktonic microfauna that they might want in their tank as invertebrate food.

So how do they keep the water clean?

Most marine aquarium hobbyists strive to maintain excellent water quality through vigorous protein skimming and frequent partial water changes. But despite this de-emphasis on mechanical filtration, many saltwater hobbyists still utilize at least some form of it.

For instance, mesh filter socks, sponge sleeves, foam blocks, and bonded filter pads are often placed in various locations within a marine aquarium’s “circulatory system” to trap particulate waste. Of course, these, too, must be rinsed or replaced frequently to prevent them from becoming biologically active and/or a repository for decomposing gunk. But it’s usually easy to do so because they’re plainly visible and you don’t have to tear down any equipment to access them.

Power filters still have a place!

Before anyone gets the idea that I’m knocking powered mechanical filters, let me assure you that they can still play an important role in maintaining good water quality in a marine system—just on more of a part-time basis.

A hang-on-back or canister filter is a great tool for those occasional heavier cleanups. For instance, if you kick up a lot of detritus during routine maintenance or when stirring the surface of your sand bed, you can temporarily hook up a mechanical filter to help eliminate this suspended debris. Similarly, if you want to “polish” your water a bit, you can always temporarily set up a power filter with activated carbon in the chamber.

I’ve used a hang-on-back filter combined with frequent water changes as part of my quarantine protocol for many years. Sometimes it takes a lot of experimentation with different foods to get a finicky new specimen to eat, and the power filter saves me a lot of trouble siphoning out uneaten food.

What’s your preference?
So, fellow salties, what’s your take on mechanical filtration in marine systems? Are you pro or con? Any interesting anecdotes to share? Let us know in the comment section below.