Reef Tank Refugium Guide: Chaeto, Lighting, Sizing, and Nutrient Export

TL;DR

A refugium grows macroalgae, usually chaetomorpha, that consumes nitrate and phosphate as it grows. Harvesting the algae removes those nutrients from the system for good. It is natural nutrient export with no dosing and no chemicals.

To work well, a refugium needs three things: enough size relative to your display, a strong dedicated light on a reverse photoperiod, and enough life in it to stay healthy. Get those right and it can carry a surprising amount of a tank’s nutrient load.

What a refugium actually does

A refugium is a calm, lit section of your sump, or a separate vessel, where you grow macroalgae on purpose. The point is nutrient export. Nitrate and phosphate build up in every reef from feeding and waste, and a refugium turns that surplus into algae growth instead of letting it fuel nuisance algae in your display.

It also does quieter secondary jobs. It shelters copepods and amphipods that breed in safety and drift into the display as live food, it can buffer pH when it is lit at night, and it adds biological stability by giving the system more surface area and life. But nutrient export is the headline.

Sizing and where it goes

Bigger is better, within the limits of your space. A refugium works by growing algae, so the more macroalgae you can light and hold, the more nutrient it can export. A common starting point is a refugium in the range of 10–20% of your display volume, and a heavily fed or stocked tank benefits from the higher end.

Most reefers run the refugium in a section of the sump, but a hang-on-back refugium or a separate lit vessel plumbed inline works too. Wherever it sits, give it gentle flow so detritus can settle and the algae is not blasted, and enough depth to hold a real mass of chaeto.

A quick reference for setting up a reef tank refugium.

Lighting and the reverse photoperiod

Light is what drives a refugium. Weak lighting is the most common reason a refugium underperforms, because the algae simply does not grow fast enough to export much. Use a strong, dedicated refugium light with plenty of output, and mount it close.

Run that light on a reverse photoperiod, meaning it is on while your display lights are off. There is a nice chemistry benefit here. Reef pH naturally falls at night because everything keeps respiring while photosynthesis stops. A refugium lit at night keeps photosynthesizing, consuming carbon dioxide and releasing oxygen, which softens that overnight pH dip and keeps the whole system steadier.

Choosing macroalgae: chaeto and the alternatives

Chaetomorpha, usually just called chaeto, is the default for good reason. It is a coarse, tangled green macroalgae that grows fast under good light, does not attach to your rock, and does not go sexual and crash the way some macroalgae can. It also holds an enormous amount of pod life in its tangle.

Other options exist. Ulva and gracilaria grow well and double as fish food, and some reefers run them alongside chaeto. Caulerpa grows aggressively but can go sexual and dump nutrients back if it crashes, so many keepers avoid it. For a first refugium, start with chaeto and keep it simple.

Keeping the refugium alive

A refugium is a living space, so treat it like one. Resist the urge to keep it spotless. A layer of detritus and a busy population of copepods, amphipods, and worms is a sign of a healthy refugium, not a dirty one, because that microfauna is part of how the system processes waste and feeds the tank.

This is also why many reefers skip heavy mechanical filtration ahead of the refugium. Filter socks pull out the very particulate and plankton the refugium and its pods would otherwise use. Let the water carry that life down, let the skimmer handle dissolved organics, and let the refugium do the rest. Give the chaeto a rinse and a shake occasionally to shed trapped detritus and keep it healthy, but do not sterilize it.

Harvesting is what exports nutrients

Here is the part people forget. Growing algae only holds nutrients, harvesting is what removes them. When you pull out a handful of chaeto and throw it away, you are physically taking nitrate and phosphate out of your system. A refugium you never harvest just holds a fixed amount of nutrient and stops exporting.

Trim the chaeto whenever it fills its space, often every couple of weeks once it is established. Take out roughly half and leave the rest to keep growing. Steady, regular harvesting keeps the algae in its fast-growth phase, which is when it exports the most.

Reading the nutrient response

A refugium works slowly and steadily, which makes it hard to judge by eye. The way to know it is doing its job is to watch your nitrate and phosphate over weeks, not days. If they hold steady or drift down as the chaeto grows, the refugium is carrying its share.

That is a lot easier to see when your tests live in one place. Reef Trak keeps your nitrate and phosphate history on a single timeline, so you can watch the trend after you get a refugium going and confirm it is actually exporting. You can log each harvest as a maintenance note too, which gives you context when the numbers move. Some reefers know it as ReefTrak without the space. Love Your Reef, Trak it.

Final take

A refugium is one of the most reliable, natural tools in reef keeping. Size it generously, light it strongly on a reverse photoperiod, grow chaeto, keep it alive rather than spotless, and harvest regularly. That last step is the one that actually exports nutrients, so do not skip it.

Run alongside sensible feeding and a good skimmer, a healthy refugium can keep nitrate and phosphate in a good range with very little intervention, and it pairs naturally with dosing-led methods like the Triton Method for reefers who want to reduce water changes.