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Back to Basics: a Look at Filtration Part 2, Mechanical Filtration

Written by Karen Randall

In my last column, we took a look at biological filtration.  This month, let’s examine the various types of mechanical filters available.

The main purpose of a mechanical filter is to remove particulate matter from the water in your tank.  There are a number of different pieces of equipment on the market that will do this job.  Most can also be fitted with various forms of chemical filtration as needs dictate.  In addition, simply because they provide attachment points for nitrifying bacteria, begin to function biologically after they have been set up for a while.

Probably the simplest and least expensive is the air driven internal box filter.  This is a small plastic container with a small tube in the middle of it.  When attached via tubing to an air pump, an air stone in the bottom of the filter draws water in through slits in the box, and back out the top.  The filter is packed with some type of fibrous material, usually poly wool, or gravel.  As the water passes through, the filter media traps particulate dirt.

Box filters are a cheap effective way to keep water clean in a small lightly populated tank.  They are very useful in hospital tanks and breeding tanks because the flow through the filter and the turbulence in the tank can be adjusted to suit the needs of the fish.  They are usually not sufficient in a large or heavily loaded system, however.  Another disadvantage is that they must be removed from the tank for frequent cleaning.  This can be a wet, smelly task, as well as disruptive to the tank.

The next step up would be a power filter of one sort or another.  Internal power filters are available from several companies.  I have one running in one of my tanks, and it seems to do a good job.  But I am not sure what the benefits are of using this filter over an external power filter.  It certainly moves a much greater volume of water than an air driven box filter, but like the box filter, it must be removed from the tank for cleaning.

Mine has the additional disadvantage of coming with a complicated bracket arrangement for holding the filter at the right depth in the water.  This required more direction reading and brain power that I usually want  to invest in installing a filter in a tank.  I also suspect that with the motor sitting down in the tank, it is heating up the water to some extent.  Whether this is a problem or not would depend on the type of fish in the tank, and the surrounding room temperature.

You will also find a myriad of external power filters on the market.  These filters hang on the back of the tank, with very little showing inside the tank.  Some are self-priming.  In these, the pump sucks water up into the filter.  The water then over-flows back into the tank after passing through the filter media.  In others, the water enters the filter via siphon tubes, and the pump then sucks the water through the filter media and returns it to the tank.

These filters have a lot to recommend them.  They are quiet and efficient, they are easy to operate, and unobtrusive in a display tank.  They come in a large range of sizes, from those small enough for a five gallon tank, to those large enough to easily handle a 70 gallon community.  Most are also easy to clean without disrupting your entire tank.

However, a power filter is not suitable in all applications.  The strong current, (and suction) may be too much for fish that are spawning, and for small fry.  The siphon type must be protected from large cichlids, who might dislodge the siphon tubes and run the filter dry, ruining the motor.  Finally, these filters, particularly the larger ones, are not cheap.  By all means invest in one for your display tank, but find a less expensive way to filter your “project” tanks.

At the top of the ladder both in price and ability to remove suspended matter from the tank are the canister filters.  Again, they are made by a number of manufacturers, and in a variety of sizes.  Generally, these filters sit below the tank, and are connected to the tank by plastic hoses, one of which moves water to the filter, while the other sends it back to the tank.  They can be filled and any of the traditional filter materials.  Some are available that run in series, with a different filter medium in each canister.

Many canister filters also offer the option of micron filtration or diatom filtration.  This is extra fine filtration for “polishing” the water.  This feature is useful on occasions where you need to immediately return a tank to display condition after working on it.  The filter cannot function in this mode long term because it clogs up quickly since it filters even the tiniest particles from the water.

Canister filters are not suitable for smaller tanks, as they are capable of moving tremendous quantities of water, that would turn a small tank into a maelstrom.  If your tank is smaller than 50 gallons, you probably can find a better type of filtration.  If, however, you are running a large display tank, or a tank for large messy fish, this could be the answer to your problems.  There is very little showing inside the tank, and they really do a great job of keeping the water clean.  Some canister filters can be a little tricky to start, but with practice are not much trouble.  Many available now are self priming, eliminating that problem.  Also, protect the rug the first few times you clean these filters because they can be messy until you’re used to dealing with them.