To begin with let’s pronounce it right. Re-few-gee-um is correct. Ref-you-gee-um is not. The very simple idea of a refugium is to improve water quality by using live, growing seaweed (aka caulerpa). Your aquarium is a living, breathing ecosystem. There is a daily balance of oxygen and carbon dioxide, pH, nutrient uptake and expulsion, temperature rise and fall and most obvious, increase and decrease of light. A refugium can help stabilize the first three. Fish breathe they take up oxygen and expel carbon dioxide the result is the carbon dioxide is absorbed by the water which changes to carbonic acid which it turn stresses alkalinity and lowers pH. Fish, corals, inverts, and bacteria all consume food in more than one way but the outcome is generally the same waste byproducts form. The most common two you would recognize are nitrates and phosphates.
The logic behind a refugium is simple. You grow live plants which require carbon dioxide, nitrates and phosphates just like your lawn. The result is a decrease in nitrate/phosphate levels and the daily pH swing (lowest in the morning highest when the lights go out) becomes minimal, As the seaweed grows, just like your lawn, it needs to be mowed. Take out 50% on a regular schedule. Should you let the sump overgrow, it will often turn white (asexual) and cloud up your tank. If this happens pull out the dead seaweed, tune up the skimmer, change filters, run carbon, and change water. Don’t panic things seldom die when this happens unless you ignore it. Restart a new bunch of seaweed and don’t make the same mistake twice.
My refugium is simple, inexpensive and easy. I use my sump only no extra tank. (See photos) First you shouldn’t have any bioballs, mud, miracle or not, substrate, grung, rubble, large rocks or similar debris in the sump. They trap sediment missed by your filter pad or bag (that you’re supposed to change at least twice weekly.) The caulerpa you need to grow (we give it away free) is rubber banded to a single piece of rock. With time it attaches and plant runners will just float around the sump. A simple intake screen will keep it out of your pump. I light my sump 24/7 using a modified outdoor flood light, about 65 W. My sumps don’t have any baffles, dividers, sections, chambers, wiers, or bubble breaks. Obstructions only slow water flow and collect sediment and are not useful.
A brand new filter / refugium setup with overhead light on. Note there is no heater or bioballs.
see Why no heaters
A working refugium with overhead light on and a lot in the skimmer cup. Note the blue tape. I marked my maximum fill level to prevent overflow during a power outage.
A working refugium with overhead light off. The calupera is free to start yours. The fan over the sump cools the system during warm weather
When your sump gets dirty, lift out the caulerpa on the rock, clean the sump and then put it back. If your nitrate levels are extremely high, the caulerpa will “nitrate burn” and die. Do 50 to 90% water changes until levels are 50 ppm or less and try again. Flow rate is unimportant in this situation. I put a power head in mine to prevent detritus from settling The idea of growing copepodes to feed your tank really doen’t work either. They are not free swimming and stick to the rocks. If they do get to the pump they will get killed