Aeration, Gas Exchange and Protein Skimming

Gas exchange happens at the water's surface: oxygen comes in from the air, and excess carbon dioxide (CO2) goes out. Fish, and even more so bacteria, use oxygen all the time, and gas exchange replaces it.

Moving the water speeds this up. Surface movement and circulation keep carrying oxygen-rich water away from the surface and bringing new water up to it (Module 13). Air pumps, flow pumps, the return and the skimmer all add to it. Warm water holds less oxygen (Module 5), and CO2 in the water pushes pH down (Module 6).

Fine bubbles give more surface for air to meet water, and their rise moves water up to the surface, where gas exchange happens. Place a stone near the bottom of the tank or sump, away from a filter outflow. Stones come in standard and fine-pore ceramic, and other kinds; wooden diffusers give very fine bubbles but clog quickly.

Does it need air? Think it through; don't follow a habit. Everything living in the water uses oxygen, and oxygen comes in only where water meets air. Ask what's in the water, how much of it, how warm it's (Module 5), how long it stays there, and whether its water is being exchanged with an aerated tank. A container, a bucket or a bin is no different from a tank: if it needs air, give it air, with the valve set to suit what's in it.

A diaphragm pump has one moving part: a rubber diaphragm flexed by an electromagnet. It flexes thousands of times a minute, so the rubber slowly hardens and cracks. A linear piston pump uses pistons instead, and is quieter and more reliable. One linear piston pump rated at 47 L of air per minute can feed about 47 tanks at about 1 L per minute per stone, and a second pump can be added to the same loop.

Stand the pump on a solid surface with free air around it, on a foam mat to cut vibration. If the pump sits below the tank rim, every line needs a check valve (section 5).

A straight connector joins two lengths of airline. An elbow turns a corner without kinking the line. A T connector and a Y connector each split one line into two; a 4-way connector splits one line into three. An inline valve sits in a line and sets how much air goes down it. A gang valve is a block with a valve on every outlet. A check valve lets air through one way only (section 5).

Every split shares the same air from the pump, so each outlet you add weakens the others. A T, Y or 4-way has no valve: the air goes wherever it escapes most easily, so put an inline valve on any branch that needs its own setting. Fittings are barbed to match the tubing; standard airline in the US is 3/16 inch (about 4.8 mm) inside. Push the tubing fully over the barb. Under the material rule, fittings and valves are plastic, titanium or 316 stainless (Module 11).

If the pump stops, water can siphon down an airline, out of the tank, into the pump and onto the floor. A check valve lets air flow toward the tank and stops water flowing back. Fit it outside the water, close to the tank rim, so water stops at the rim. The end with the flapper faces the air pump; fitted backwards, no air flows at all. A check valve isn't needed if the pump sits higher than the tank rim.

An air valve sets how much air goes down a line. A gang valve splits one pump between several outlets, each with its own valve; gang valves can be chained for more outlets. Every split shares the same air, so each outlet you add weakens the others. Under the IMARCS material rule, valves and fittings are plastic, titanium or 316 stainless, not brass (Module 11).

A manifold is anything that takes air from one pump and splits it to many outlets. It can be simple: a molded plastic bar with one inlet and a row of 12 outlets, which is how IMARCS often builds them (Michael Maddox, President of IMARCS). It can be a gang valve, or several chained together. It can be a pipe with outlets fitted into it, from 1 to 2 cm (about 3/8 to 3/4 inch) pipe on a bench up to a loop of 25 mm (1 inch) pipe around a fish room, starting and ending at the pump.

Choose by thinking it through. How many outlets? How far apart are they? Does each one need its own setting? The pipe or block must carry enough air for every outlet it feeds, and allow about 1 L of air per minute per stone when choosing the pump. Put a valve on any outlet that needs its own setting; a molded manifold with no valves can feed lines that each carry an inline valve. Cap or close outlets not in use. If the outlets sit below the tank rims, every line gets a check valve.

For a rack split into compartments, a bank of small tanks or a quarantine row, give every compartment or tank its own outlet and valve, and label each valve with what it feeds, the same way water manifold valves are labeled (Module 12). Then any one outlet can be checked or shut without guessing.

Open the valves in use, close unused outlets, then adjust one valve at a time until each stone gives the bubbles it should. After changing any valve, check every outlet again. If a second pump is added, rebalance.

With only a few outlets on a big pump, there can be too much air. Bleed the extra off through a spare valve to a stone held outside the water, rather than closing everything down.

Work through it in order, changing one thing at a time (Module 1). Start at the stone: pull it off its line. If air flows from the bare line, the stone is clogged. If not, move back one joint at a time toward the pump, checking each valve and fitting.

Common causes: an open port on a gang valve or loop with no line on it, where the air escapes; a valve left open on an unused outlet, bleeding off pressure; the valve to this stone closed; a check valve fitted backwards; a kinked, split or loose airline; a clogged stone, giving fewer and larger bubbles; and the pump itself. A pump that runs weak, louder or hot has a worn diaphragm or a clogged intake filter; one that gives no air at its outlet has a torn diaphragm.

To fix it: fit a line to the open port or close it; close the stray valve; open the right valve and rebalance (section 7); turn a backwards check valve round; replace a damaged line; replace or soak the stone; rebuild the pump's diaphragm (section 9). Then check every outlet again.

Rebuild the diaphragm every 18 to 24 months in normal conditions, and every 12 to 18 months where it's hot, dusty or humid. Signs: weaker bubbles, louder running, a hot housing, or no air at all. A rebuild kit holds new diaphragms and an air filter. Unplug the pump, open it, note how every part sits, fit the new diaphragms the same way, snug but not overtight, replace the intake filter and test it. A clogged intake filter alone can make a pump underperform.

Replace air stones every three to six months. A clogged stone can be soaked in white vinegar for 24 hours and rinsed thoroughly.

A protein skimmer uses foam fractionation. It mixes air into tank water as very fine bubbles. Dissolved organic molecules stick to the bubble surfaces, the bubbles rise and form foam, and the foam spills over into a collection cup, taking the waste out of the system. The smaller the bubbles, the more surface they give, and the better the skimming.

Follow the diagram. The needle-wheel pump pulls in sump water and, through the air line and its silencer, room air; its pinned impeller shreds the air into fine bubbles and pushes the mix into the reaction chamber. The bubbles rise, collect waste, and become foam in the cone and the neck. The foam climbs the neck and spills into the cup as dark liquid called skimmate. Cleaned water leaves through the water outlet, whose height sets the water level inside the skimmer.

A skimmer removes proteins and amino acids from food and waste, fats and carbohydrates, some metals and trace elements such as copper and iodine, and particles, phytoplankton, bacteria and detritus. A skimmer sits in the sump or hangs on the back of the tank, depending on its design. Venturi, downdraft and Beckett designs also exist.

Set the skimmer in the water depth its maker recommends. Start with the air intake low and raise it gradually until the foam is steady. Foam that's too wet removes a lot of water with little waste; foam that's too dry may not remove enough. Water flow through the skimmer matters too: too fast and waste can't attach to the bubbles, too slow and it skims less.

A new skimmer takes about 2 to 4 weeks to break in, and can overflow or behave erratically until it settles. While a new system is cycling there's little waste to skim: the skimmer can be turned off, or left running for its aeration with the cup removed.

Causes: a new skimmer still breaking in; the sump water level too high; water conditioners, bacteria products, oily foods, medications or coral epoxy in the water; residue from new filter socks; household cleaners or air fresheners sprayed near the tank; or a dirty pump or blocked air line.

To fix it: set the sump level to the maker's depth. For anything added to the water, run fresh activated carbon and do extra water changes; while medicating, the skimmer may keep overflowing. Rinse new socks before use (Module 15). Clean a dirty pump and clear the air line (section 13).

Empty and clean the collection cup regularly, and wipe the cup's neck, where the foam rises; a dirty neck stops consistent skimming. Take the skimmer apart and clean the body periodically.

To clean the pump: unplug it, take it out and take it apart. Soak the parts in citric acid and water, 2 to 3 tablespoons (30 to 45 mL) per gallon (3.8 L). Never soak a pump in vinegar. Scrub with a toothbrush, rinse in RO water and refit (Module 13).

The needle wheel: pull it out and clean between its pins with a brush. Look for broken pins, cracks or a loose magnet; a damaged wheel is replaced, not repaired (Module 13).

Air line blockages: salt can crust in the air intake, the silencer and the venturi, and less air in means less foam. Pull the line and silencer, rinse them in warm fresh water, clear the venturi and refit.

Closed loop scrubber blockages: on a closed loop scrubber (section 14), the skimmer's air runs through the scrubber and back. Wet or clumped media, water in the lines or a kinked line chokes the air, and skimming drops. Replace wet or clumped media, keep the RO water off the media, drain and straighten the lines, and check the fresh air valve.

Indoor air, especially in a closed room with people breathing, can hold enough CO2 to push a tank's pH down (Module 6). A CO2 scrubber is a canister of CO2-absorbing media fitted to the skimmer's air intake, so all the air the skimmer draws in passes through the media first.

An open scrubber draws room air through the media. A recirculating (closed loop) scrubber uses a skimmer lid that sends the skimmer's exhaust air back through the media; valves set the mix of recirculated and fresh air. Recirculated air has already lost most of its CO2, so the media lasts longer.

Color-changing media shows when it's used up. Replace it when most of it has changed color, or when pH starts to drop. That's often about every 2 weeks, depending on the scrubber's size and the skimmer. A tablespoon of RO water in the bottom of the housing keeps the air humid; keep that water off the media.

An ozone generator makes ozone from air with electricity. It isn't an air pump: suction from the skimmer or reactor draws the gas through. An air dryer before it removes moisture, which can cut ozone output by as much as half; its beads change color when spent and can be recharged by baking at about 150 C for 3 hours. Mount the generator above the water with an ozone-safe (Kynar) check valve, and use ozone-resistant tubing, since ozone breaks normal tubing down fairly quickly.

Ozone goes in one of two ways. Through the protein skimmer: connect it to the skimmer's air inlet, venturi or ozone port, and the skimmer becomes the reaction chamber. Or through a dedicated ozone reactor, a closed chamber where ozone meets tank water, or a second skimmer used only for ozone. Either way, the air leaving it passes through activated carbon to catch leftover ozone, and that carbon is replaced regularly.

A starting point is 15 mg of ozone per hour per 95 L (25 gallons) of water. Ozone raises ORP (Module 6): keep it at 300 to 350 mV, never above 400. Let it settle for a few days after any change, and change the dose in small steps. Too much ozone harms or kills fish and invertebrates, and ozone in the room irritates the lungs. If you smell ozone: unplug the generator, open a window and replace the carbon.