Saltwater Aquarium Refugium and Sump Design Guide | The Ultimate Water Quality Stabilizer

Sumps (filter tanks) and refugiums (ancillary tanks where macroalgae thrive) are powerful allies for maintaining stable saltwater aquarium water quality over the long term. Installed beneath the main tank, this system offers numerous benefits: increased water volume, consolidated equipment, and improved biological filtration capacity. This article provides a detailed explanation of sump and refugium design through operation.

Basic Concepts of Sumps and Refugiums

A sump is an auxiliary tank installed beneath the main tank and connected to the main tank via an overflow system. There are three major benefits to using a sump.

1. Increased total water volume reduces the likelihood of sudden water quality fluctuations. For example, if the main tank is 200L and the sump is 100L, the system's total water volume becomes 300L.
2. Equipment such as protein skimmers, heaters, and automatic top-off systems can be consolidated in the sump, keeping the main tank's appearance clean.
3. Utilizing part of the sump as a refugium enables biological water purification.

A refugium, meaning "refuge," is an area where macroalgae and microfauna (small crustaceans and microorganisms) are cultured.

Sump Baffle Design

Typically, the interior of a sump is divided into multiple sections using baffle plates. Here is an overview of a basic three-chamber configuration:

Baffle plates can be constructed by bonding acrylic or glass with silicone, and DIY construction is possible.

Selecting and Managing Macroalgae in the Refugium

The standard macroalgae for refugium cultivation is Chaetomorpha (commonly called cheto). A green filamentous algae, it has a high capacity to absorb nitrate and phosphate, and is easy to manage. It grows under light, and requires no special maintenance beyond occasional thinning if it overgrows. Caulerpa is also popular, but carries the risk of a "crash"—simultaneous die-off during sexual reproduction that rapidly deteriorates water quality—so 24-hour lighting is needed to suppress reproduction. For beginners, Chaetomorpha is highly recommended. Refugium lighting should run on the opposite cycle from the main tank (refugium on while main is off), which helps suppress pH fluctuations. Since macroalgae absorb CO2 during photosynthesis and raise pH, this compensates for nighttime pH drops.

Microfauna Reproduction and Utilization

Another major benefit of the refugium is the natural reproduction of microfauna such as copepods (a type of small crustacean) and amphipods. These tiny crustaceans are supplied to the main tank via the return pump and serve as natural live food for fish. For fish that prefer microscopic prey, such as mandarin fish and flame wrasses, the natural food from the refugium is an invaluable nutritional source. To promote microfauna reproduction, place small pieces of live rock or ceramic ring media in the refugium to provide habitat. It is crucial not to place fish in the refugium—if fish are present, the microfauna will be consumed entirely and the reproduction cycle will collapse.

Water Flow and Return Pump Selection

The return pump is the heart of the sump system. A general guideline for pump flow rate is 5 to 10 times the main tank's volume per hour. For a 200L main tank, select a pump rated for 1,000–2,000 L/hour. However, actual flow decreases with head pressure (the height water must be lifted), so choose the pump based on flow accounting for your head. DC pumps are increasingly common because they offer adjustable flow and low power consumption. It is important that the overflow's drainage capacity matches the return pump's flow rate—if return flow exceeds the overflow's drainage capacity, the main tank will overflow. Conversely, account for water backflow from the main tank to the sump during a power outage, and ensure the sump has adequate reserve capacity. Installing a check valve to prevent backflow and a siphon break hole are also important safety measures.

Refugium (Macroalgae Filter) and Sump Design Guide

Refugiums and sumps are systems that play an important role in saltwater aquarium water quality management. This section explains their respective functions and design considerations.

What is a Sump

A sump is a filter tank installed beneath the main tank that houses equipment such as protein skimmers, heaters, dosing pumps for additives, and activated carbon reactors. The primary benefit is maintaining a clean appearance in the main tank. The ideal sump capacity is 30–50% of the main tank's water volume; for a 60L tank, a 20–30L sump is appropriate.

Refugium Design and Macroalgae Selection

A refugium is a section created by partitioning part of the sump where macroalgae are cultivated. Since macroalgae grow by absorbing nitrate and phosphate, nutrient levels can be reduced through methods other than water changes. The most popular macroalgae is Chaetomorpha (Tamimiru), which multiplies rapidly and is easy to manage. Sea grape (Caulerpa racemosa) is also popular, but carries the risk of sudden water quality changes if sexual reproduction (mass spawning) occurs, so control proliferation through regular thinning.

Dedicated refugium lighting (LED plant-growth fixtures) running 24 hours or on the opposite cycle from main tank lighting also helps mitigate nighttime pH drops.

Copepod and Microfauna Reproduction

Another major benefit of the refugium is the natural reproduction of small crustaceans such as copepods (calanoids) and amphipods (side swimmers). These serve as natural food for fish and corals and are essential for keeping mandarin fish and seahorses.

Installing a refugium is an investment that greatly enhances the tank's ecosystem. Beyond simply absorbing nutrients, the macroalgae enable natural reproduction of small crustaceans such as copepods and amphipods, which become natural food sources for fish and corals. Operating refugium lighting on the opposite cycle from the main tank lighting also helps mitigate nighttime pH drops. Another major benefit is the ability to physically export nutrients by regularly thinning the macroalgae.

Create a Thriving Saltwater Aquarium with BreederDirect

Implementing sumps and refugiums makes saltwater aquarium maintenance significantly easier and creates a more comfortable environment for aquatic life. At BreederDirect, you can consult with breeders of saltwater fish and corals about system design and receive advice on sourcing macroalgae for your refugium. We encourage you to enjoy vibrant saltwater fish thriving in stable water conditions.