Mechanical Filtration

Mechanical filtration removes particulate: anything solid carried in the water. Some of it's living, like tiny algae, bacteria and crustaceans. Some is dead: waste, dying plankton and leftover food. Some is mineral: crumbs of rock, shell and coralline algae, and scrapings from the glass. Detritus is the dead organic part.

Left in the system, waste and leftover food break down and raise nitrate and phosphate. Particles clog chemical media, scatter light before it reaches the corals, and settle as sludge in still spots. Sediment passing through pumps wears their shafts and bearings. Detritus can also feed cyanobacteria outbreaks and kill coral fragments from the base.

A mechanical filter only helps if what it catches leaves the system, so it has to be pulled out and cleaned or changed on time. Good flow keeps detritus in suspension so it reaches the filters (Module 13).

A micron is one millionth of a meter, a thousandth of a millimeter. A filter's micron rating is the average size of its openings. A human hair is about 70 microns across; the eye can just see about 40.

The smaller the rating, the smaller the holes: a 100 micron sock catches finer particles than a 200 micron sock, and clogs faster. Most socks are rated 200 or 100 microns. As a filter fills, it catches smaller and smaller debris, and the water starts to back up.

Most bags and socks carry a nominal rating: some particles larger than the rating still pass, and media is typically rated at 80% efficiency. An absolute rating catches 99.9% of particles at that size, and costs much more.

Water from the overflow drain (Module 12) passes through the mechanical filter first, before the protein skimmer and before any chemical or biological media. Caught early, particles can't clog the media behind them or reduce the skimmer's foaming.

Where a system uses ozone, it's usually mixed into the water in the protein skimmer, and the water leaving it passes over activated carbon to remove leftover ozone. A UV sterilizer, if used, goes after the mechanical filter and before the biological media, usually on its own pump drawing from an early sump chamber and returning to a later one, so the same water isn't treated over and over. UV only affects what's suspended in the water and passes through it. Both are covered in Module 21.

In a canister or a stack of pads, the same rule applies: coarse first, fine last, and mechanical before biological. The aim is that most of the drain water passes through the filter, not around it (section 12).

Most socks have a rigid plastic ring that sits in a sock holder in the sump; common sizes are 4 inch and 7 inch rings, the larger for higher flow. Drawstring socks tie onto the end of a pipe where there's no holder.

Felt socks catch more debris but clog faster and are harder to clean. Mesh socks clog less quickly and are easier to clean, but catch less. Felt is the usual choice in a reef. Socks are polyester felt on plastic rings, which fits the material rule (Module 11).

Socks catch leftover food as well as waste: anything solid the drain carries. IMARCS rule (Michael Maddox, President of IMARCS): change filter socks every 3 days, and sooner if there's a lot of waste.

Change socks every 3 days (section 4), sooner if there's a lot of waste, and as soon as one discolors. If water is overflowing the top, it should have been changed already. A clogged sock holds concentrated waste that keeps breaking down in the water flowing past it.

To change a sock: pull the used sock and put a clean one in. Turn the used sock inside out so the waste side is outward, then wash it. There are several good ways to wash socks:

• Dishwasher. IMARCS practice (Michael Maddox, President of IMARCS): turn the sock inside out, wring it out, and put it in a dishwasher kept only for washing socks.
• Washing machine. Run a hot rinse cycle with nothing in it first, then wash the socks hot. Use only fragrance-free detergent or fragrance-free bleach (two or three capfuls), and rinse thoroughly.
• Bleach soak. Soak the socks in hot fresh water with 1 to 2 capfuls of fragrance-free bleach per gallon (about 4 L) for a few hours or overnight, then rinse them inside and out.
• Hose or power washer. With the sock inside out, rinse or blast both sides until clean. Watch the pressure: too much tears the fibers.

IMARCS rule (Michael Maddox, President of IMARCS): whatever the method, any detergent or bleach must be fragrance-free, and the socks must be rinsed thoroughly before they go back in a system. After any method that uses bleach, let the socks air dry for at least 2 full days; if they still smell of bleach, soak them in water with dechlorinator. Replace a sock when holes wear through or the stitching comes apart. Keep enough socks on hand that a clean one is always ready.

Rinse new socks before their first use; residue from manufacturing can make the skimmer overflow. If it does: turn the skimmer off, add fresh carbon, do a water change and wait 24 to 48 hours before restarting it. Filters also fill faster after maintenance that stirs up detritus, such as blowing off rock, so check them afterward.

Drain water flows through a strip of fleece stretched between a clean roll and a take-up roll. As the fleece clogs, the water level rises. A float or optical sensor sees the rise and runs a motor that winds the used fleece onto the take-up roll, bringing clean fleece under the water. Used fleece is thrown away, not reused, unless its maker says otherwise.

How long a roll lasts depends on its size, how many animals the system holds, feeding and how much debris there is. On the Red Sea ReefMat 500, a 28 m roll of polyester fleece lasts roughly 5 to 8 weeks on a moderately stocked 80 gallon reef. Bare-bottom tanks use fleece faster, and so do the first weeks after installing a roller and any maintenance that stirs up detritus.

A roller has an emergency overflow that lets water escape if the fleece can't advance. Keep it clear. Don't exceed the maker's maximum flow, and leave room to change the rolls.

IMARCS rule (Michael Maddox, President of IMARCS): if a fleece roll isn't advancing, water overflows the roller chamber. An emergency overflow that runs often is the warning sign: something is stopping the fleece.

To fix it: clean salt creep, algae and debris off the water-level sensor. Check the fleece is routed correctly, with no folds or tears, and that the used fleece winds on evenly. Check there's clean fleece left on the roll; if it's run out, fit a new roll. Then check the motor advances the fleece when the sensor trips. Then test that it advances before leaving the system unattended.

Check the clean fleece left on the roll during routine work, so the roll is replaced before it runs out. After installing a roller or a new roll, test the sensor and motor before leaving the system unattended, and watch it closely for the first few days.

Filter floss is fine polyester fiber. Its fine fibers trap even tiny particles, which polishes the water clear. Pads come from coarse to fine; coarse pads catch larger debris and protect the finer media after them.

In a reef, replace fine floss rather than rinsing it: it loses its structure after one use, and the waste it holds starts to break down quickly. Pads around a busy overflow may need changing every few days. A bad smell from the media means trapped waste is rotting; change it more often.

Pack floss evenly. Too tight cuts the flow and makes the pump work harder; too loose lets water through the gaps unfiltered. Use only floss sold for aquariums, or 100% polyester with no dyes, fire retardants, bonding agents or other additives.

Foam catches larger debris and works well as a pre-filter in front of finer media. It's sold by PPI, pores per inch, such as 20, 30 or 60; more pores per inch means smaller pores and finer filtering.

IMARCS rule (Michael Maddox, President of IMARCS): mechanical filtration must never serve as biological filtration. They're two separate things. Mechanical media is there to catch waste, so it's pulled out quickly and cleaned completely, not left in to grow bacteria. Biological media is covered in Module 8.

To clean foam: pull it out every 3 days or more often, before the waste it holds breaks down, and clean it completely. Replace it when it starts to fall apart.

In a canister, water passes coarse foam, then fine foam, then any chemical media, then biological media, with polishing floss as an optional last stage. The main concern on a reef is that a canister traps organic waste. Pull the mechanical pads out often and clean or replace them completely. The flow drops as a canister gets dirty; never let it fill with sludge.

A micron cartridge is a pleated filter element that removes very fine debris, leaving the water very clear. Some can also be charged with diatom powder. Rinse the cartridge under pressure when the flow drops a lot, and replace it if it won't come clean; never brush or machine wash it. Keep a second cartridge so one can run while the other is cleaned.

IMARCS practice (Michael Maddox, President of IMARCS): internal canister filters with micron cartridges are run during spawning events. Never run a canister dry, keep its plug dry, and give the cord a drip loop (Module 2).

Marine ich (Cryptocaryon irritans) shows as white spots on fish. Its feeding stage lives on the fish for about 3 to 7 days, then drops off and forms a cyst on a surface in the system. Each cyst releases about 100 to 1,000 free-swimming theronts, usually 4 to 8 days later, which look for a fish. A theront is about 20 to 30 microns wide and 50 to 70 long. Its ability to infect drops sharply 6 to 8 hours after it leaves the cyst. The full cycle is typically 1 to 2 weeks, but can run from 6 days to 11 weeks.

Marine velvet (Amyloodinium ocellatum) is another parasite of marine fish. Its free-swimming stage, the dinospore, is much smaller: about 8 to 15 microns across. Dinospores lose most of their ability to infect after about 24 hours, but can stay infective for at least 6 days.

IMARCS guidance (Michael Maddox, President of IMARCS): for marine ich, temporarily switch to 10 micron filter socks to help eradicate it; for marine velvet, use 5 micron filter socks. The target is the free-swimming stage, and any other stage that water motion keeps suspended. The stage on the fish can't be caught by mechanical filtration.

Particles smaller than a filter's rating pass through it, so 100 and 200 micron socks let theronts and dinospores through. A 10 micron sock is rated well below a theront's 20 to 30 micron width, and a 5 micron sock below the 8 to 15 microns of velvet's dinospores. A filter only catches what passes through it, the same way a UV sterilizer only affects parasites that pass through the unit, and cysts on the rock aren't touched. Fine filters clog faster, so check and change them more often while they're in use, then go back to the normal filter. Diagnosing and treating disease is covered in Module 30, and quarantine in Module 23.

As a sock or fleece clogs, the water level behind it rises. A clogged sock overflows at the top, so most of the water goes over it instead of through it. A roller that can't advance sends water over its emergency overflow. Either way the filter has stopped working.

A gap between a sock holder and the sump wall, or a sock that doesn't seal in its ring, lets water go around the sock. The sign is detritus building up in the sump.

To fix it: change a clogged sock or fleece right away. For a gap at the holder, close it with a removable strip of plastic foam, such as polyethylene backer rod, or reseat the holder if it clips in. Never block the emergency overflow itself: it's the path the water takes when a filter clogs, instead of overflowing the filter chamber.

Filter socks are simple, cheap and need no power, but they must be changed by hand, often, and they stop working when a change is late. Fleece rollers advance clean fleece by themselves and keep filtering steadily with less hands-on work, but they cost more, need power and replacement rolls, and take sump space.

Floss and pads fill canisters and can be wrapped around overflow pipes; in a reef they're replaced often. Micron cartridges give the finest polishing and suit temporary use, such as during a spawning event. Against parasites, IMARCS switches to 10 micron socks for ich and 5 micron socks for velvet (section 11).

Whichever method a system uses, regular changes and checks are what make it work, with spares on hand and the emergency overflow kept clear (Module 32).