UV and Specialized Equipment

A UV sterilizer pumps water past a lamp that gives off UV-C light, inside a quartz sleeve that keeps the water off the lamp. UV-C damages the DNA of organisms suspended in the water passing through, so they can't reproduce. It reduces free-floating populations; it doesn't wipe them out, and it removes nothing from the water.

Only what passes the lamp is affected. Bacteria living on rock, sand and filter media stay on those surfaces and aren't exposed, and neither are fish, rock or glass. Everything drifting in the water is exposed, including plankton and the spawn of invertebrates (section 6). Like low-micron filtration (Module 15), UV can only reach the free-swimming stage of a parasite, never the stage on the fish. Module 16 covers ozone, the other oxidizing treatment, and Module 11 covers which materials stand up to UV.

UV dose is measured in microwatt-seconds per square centimeter (µWs/cm²). Free-floating algae and bacteria need a lower dose, about 30,000 µWs/cm²; protozoa, which include parasites, need about 180,000 µWs/cm². The dose depends on how long water spends in front of the lamp, so slower flow means a higher dose.

Makers give two flow rates: a faster one for algae and bacteria and a much slower one for parasites. For example, a 25 W unit lists about 1,790 L/h (472 US gallons per hour) for algae and bacteria, but about 300 L/h (79 US gallons per hour) for protozoa. Measure the flow, don't guess: time how long the line takes to fill a 19 L (5 US gallon) bucket. One minute is about 1,140 L/h (300 US gallons per hour); two minutes is about 570 L/h.

Decide what the UV is for: clear green water and bacteria, or reduce parasites. Higher wattage means more UV and a bigger unit. Never choose a unit whose maximum system size is smaller than your real water volume (Module 36); a larger one is fine. Then set the flow to the maker's rate for your target, with a pump that can reach it against its plumbing and a valve to set it.

One unit can't do both targets at once, because each needs a different flow. Either pick one, or run two units at different flows. Some makers deliver a stronger dose at their listed flows than others, so compare the published flow and dose, not just the watts.

The most common setup is inline on the return pump line: simple, and hidden in the cabinet, but the flow is whatever the return gives. A dedicated pump with a valve, or a controllable DC pump, sets the flow exactly. A closed loop drawing straight from the display is the most effective but least convenient.

Draw water from one sump chamber and return it downstream, such as from the first chamber to the return chamber, so the UV isn't re-treating the same water. Mounted horizontally, the inlet and outlet face up; mounted vertically, water enters at the bottom and leaves at the top. Trapped air heats the unit and cuts its effect. Secure the lines so they can't fall out of the sump, and never switch the lamp on until water is flowing through it.

A UV can run all day, run on a timer for part of the day, or sit ready and run only when a problem appears. Common reasons to run one are new animals being added, green water, a disease outbreak and dinoflagellates. It reduces what's floating; it doesn't cure fish that are already sick (treatment is Module 30).

Whether to run it all the time is a separate decision, covered in the next section. Whatever the schedule, switching a UV on or off is a change to a running system: make one change at a time and watch the result (Module 1).

Reasons to run it all the time. It keeps free-floating algae, bacteria and free-swimming parasites down every hour of the day, not only after a problem shows. That matters most while new animals are being added, and in quarantine systems. It also keeps the water clear and can help against dinoflagellates.

Reasons not to. UV doesn't choose what it kills. Phytoplankton and bacterioplankton that pass the lamp die along with the nuisance species. They're a natural food for many filter feeders, corals included, and while alive they take up nitrate and phosphate, so with fewer of them nutrients can climb. Most aquariums already hold little plankton, and a UV running all day keeps it lower still. That's the cost to biodiversity: less free-floating life, and less food for the animals that depend on it.

How much that costs depends on the system. A system started with dry rock holds little invertebrate life and little plankton, so there's little for UV to take. A high-biodiversity system is built the other way: started with uncured live rock taken straight from the ocean, and fed a wide range of foods, from bacteria through zooplankton. Many hold living plankton, including spawn from broadcast-spawning invertebrates. A UV in a system like that will dampen the biodiversity the system is built on.

Switch the UV off for these:

• Medication: UV can break medications down, chelated copper in particular. Switch it off for the treatment and follow the medication's own directions (Module 30).
• Dosed bacteria: bacteria products go into the water column, where the UV reaches them. Some makers direct switching off the UV, often with the skimmer, for 12 to 48 hours after dosing, depending on the product. Follow the label.
• Live phytoplankton and rotifers: food dosed for filter feeders passes the lamp too. Feed while the UV is off, so the food reaches the animals first.
• A new system that's still cycling: UV can kill nitrifying bacteria in the water before they settle on rock and media (Module 9).

A timer is the middle ground: the UV runs part of the day and stays off while food, bacteria or medication is in the water. Log every switch-off with its planned switch-on time (Module 20), so a UV is never left off by mistake. Nuisance algae growing on rock and glass never pass the lamp, so UV doesn't replace good maintenance.

UV lamps lose output with age, and the quartz sleeve gets a film that blocks UV-C. Replace the lamp and the sleeve about once a year, following the maker. Green water coming back after months of clear water is a sign they're due. Clean the sleeve whenever the unit is open.

Safety: unplug the unit before opening it. Handle the lamp with gloves or a towel so skin oil doesn't mark the glass. UV-C harms eyes and skin; check that the lamp is on through the unit's viewing window, never by looking at a bare lamp. After reassembling, check the cap and seals for drips, let water fill the unit, then switch it on.

No glow in the window: check the outlet and controller (Module 20), then the ballast and lamp. Green water coming back after months of clear water: the lamp and sleeve are aging or the sleeve is filmed. No effect at all: the flow may be far too fast for the target, or the inlet and outlet are in the same chamber so the same water goes round and round. Unit hot or noisy: trapped air or no flow; switch it off until water is flowing. A drip at the cap: the O-rings or seals need reseating or replacing.

Measure the flow with the bucket test before and after any change, and change one thing at a time. A UV can sit dark for weeks without anyone noticing, so its glow is part of the daily equipment check (Module 20).

An automatic water changer (AWC) uses level sensors and pumps. A typical unit has two optical sensors in the chamber where the level moves (usually the return chamber): it pumps water out to a waste container or drain until the low sensor trips, then refills from a container of new salt water until the high sensor trips. Some swap water in and out at the same time instead. Many combine with the top-off, using the high sensor to add fresh water between changes. Siphon breaks and secure line holders stop water flowing back by itself.

The new water must match the system's salinity, temperature and alkalinity, just like a manual water change. The big risks are the wrong water in the new-water container, a container that runs empty, and a waste line that siphons. The top-off failsafes in Module 20 apply here too: check the containers and sensors in the daily equipment check, and watch one full change after any setup work.

Automatic feeders hold dry food and drop a set portion on a schedule. Some keep food dry with a closed drawer, use different tray sizes to set the amount, can pause the pumps during feeding, and can warn when food runs low. Food that gets damp clumps and stops dropping, so check that each feeding actually dropped.

Dry food is only part of a varied diet. Cold food feeders keep liquid or thawed frozen food in a fridge and pump a set amount into the sump on a schedule, with a peristaltic pump like a dosing pump (Module 20). The food is kept as close to freezing as possible without freezing, because unpreserved food goes bad in less than a week. Wide pump tubing and finely ground food of even size clog less. Tank water must never run back up the line into the food: it contaminates it. These feeders make it easy to overfeed, so watch nitrate and phosphate, and keep flow and mechanical filtration strong.

Feeding is also a health check. A healthy clownfish springs out and eats when food goes in, even if it was resting on the bottom. A feeder can't see that, so it adds to hand feeding rather than replacing it; someone still watches the animals eat and removes food left uneaten. Live and frozen foods are covered in Module 26.