You’ll want compact, reliable HOB refugiums that hold macroalgae, protect microfauna, and fit your tank rim. Top picks include the clear ISTA IF-648 breeder box for isolation, Ultum Nature Systems’ black HOB refugium for Dual AIOs, the adjustable-baffle HOB‑R with Sicce pump, and CPR’s AquaFuge 2 (small and medium) ready‑to‑run units with LEDs and updated outlets. Each balances flow, lighting, and maintenance differently — keep going to see which matches your setup and goals.
| ISTA IF-648 Hang-On Separation Breeder Box,clear |
| Best for Breeding | Mount Style: Hang-on (hangs on tank wall) | Primary Purpose: Separation / breeder box (protects fish/fry) | Material: Acrylic | VIEW LATEST PRICE | Read Our Analysis |
| Ultum Nature Systems HOB Refugium (Black) |
| Macroalgae Specialist | Mount Style: Hang-on-back (HOB) | Primary Purpose: Refugium / macroalgae cultivation / nutrient export | Material: Black acrylic | VIEW LATEST PRICE | Read Our Analysis |
| HOB-R Hang-On Back Refugium for Aquariums |
| High-Performance Pick | Mount Style: Hang-on-back (HOB) | Primary Purpose: Refugium / macroalgae cultivation / nutrient control | Material: Plastic (acrylic/plastic components) | VIEW LATEST PRICE | Read Our Analysis |
| AquaFuge 2 Hang-on Refugium with Pump Light Stand |
| All-in-One Starter | Mount Style: Hang-on-back (HOB) | Primary Purpose: Refugium / macroalgae cultivation / nursery | Material: Plastic | VIEW LATEST PRICE | Read Our Analysis |
| CPR Aquatic AquaFuge 2 Medium Hang-On Refugium |
| Pro-Level Grower | Mount Style: Hang-on-back (HOB) | Primary Purpose: Refugium / macroalgae cultivation / nursery | Material: Acrylic / plastic | VIEW LATEST PRICE | Read Our Analysis |
More Details on Our Top Picks
ISTA IF-648 Hang-On Separation Breeder Box,clear
In case you need a simple, space-saving breeder box that won’t block your view, the ISTA IF-648 delivers: its clear acrylic hang-on design lets you isolate aggressive or baby fish without rearranging the tank, and the air-driven operation makes water changes and maintenance easy so you can keep fish safe at every life stage. You’ll hang the IF-648 on freshwater or saltwater tanks to protect fry, weak, or territorial fish. At 5.5 x 10 x 5.25 inches and one pound, it’s compact and unobtrusive. Durable acrylic and air-powered flow mean reliable separation and straightforward upkeep for busy reefkeepers.
- Mount Style:Hang-on (hangs on tank wall)
- Primary Purpose:Separation / breeder box (protects fish/fry)
- Material:Acrylic
- Includes Pump or Air Drive:Air-driven (requires/uses air)
- Intended for Freshwater or Saltwater:Freshwater and saltwater compatible
- Ease of Maintenance / Access:Easy water changes without affecting separated fish (easy maintenance)
- Additional Feature:Clear acrylic body
- Additional Feature:Air-driven operation
- Additional Feature:Hang-on, easy water changes
Ultum Nature Systems HOB Refugium (Black)
In case you run a UNS Dual AIO setup and want a compact, easy-to-install refugium for growing Chaetomorpha and improving water quality, the Ultum Nature Systems HOB Refugium (Black) is a smart pick. You’ll hang the durable black acrylic unit on tanks up to 12 mm glass and slide it into the Dual AIO’s second chamber (UNS 40A/60A support all accessories there). It cultivates macroalgae and beneficial bacteria to reduce nitrates and phosphates, clarifies water, and supports coral health. The kit includes the refugium and a nylon screw; lighting and extra fittings are sold separately.
- Mount Style:Hang-on-back (HOB)
- Primary Purpose:Refugium / macroalgae cultivation / nutrient export
- Material:Black acrylic
- Includes Pump or Air Drive:No pump included (light sold separately; nylon screw included)
- Intended for Freshwater or Saltwater:Saltwater / reef tanks (refugium focus)
- Ease of Maintenance / Access:Separate access to refugium chamber and lighting for simplified upkeep
- Additional Feature:Fits tank walls ≤12mm
- Additional Feature:Designed for macroalgae lighting
- Additional Feature:UNS Dual AIO compatible
HOB-R Hang-On Back Refugium for Aquariums
At the point you want a compact refugium that installs without drilling and keeps maintenance simple, the AquaReady HOB-R Hang-On Back Refugium is a great pick—its hang-on-back design and removable trays let you add macroalgae and clean the unit without disrupting the display. You’ll get an adjustable baffle system that even accepts a sand bed, a large rear viewing window, and a slotted pre-outlet to stop macroalgae from escaping. The included ultra-quiet Sicce Syncra Nano pump (2.8 W) sits inside the baffle, and the outlet sponge reduces microbubbles. It fits rims up to 0.75″, weighs 8.16 lb, and carries a one-year warranty.
- Mount Style:Hang-on-back (HOB)
- Primary Purpose:Refugium / macroalgae cultivation / nutrient control
- Material:Plastic (acrylic/plastic components)
- Includes Pump or Air Drive:Pump included (Sicce Syncra Nano)
- Intended for Freshwater or Saltwater:Freshwater and saltwater compatible
- Ease of Maintenance / Access:Removable trays, removable inlet, adjustable baffle for easy cleaning
- Additional Feature:Adjustable baffle system
- Additional Feature:Included Sicce Syncra Nano
- Additional Feature:Slotted pre-outlet (algae control)
AquaFuge 2 Hang-on Refugium with Pump Light Stand
In case you want a compact hang-on refugium that’s ready to run out of the box, the AquaFuge 2 Small Hang-on Back Refugium is a smart pick — it ships with a pump, LED light, and stand so you won’t need extra accessories to get started. You’ll get a black, modern plastic unit from CPR Aquatic with a new outlet design and updated LED light. It’s aimed at fish aquariums and performs basic refugium duties without a fuss. Available since 2014, it earns high marks (4.8/5 from four reviewers). Check seller details for current price and shipping; sign-in might be required.
- Mount Style:Hang-on-back (HOB)
- Primary Purpose:Refugium / macroalgae cultivation / nursery
- Material:Plastic
- Includes Pump or Air Drive:Pump included (powerhead included)
- Intended for Freshwater or Saltwater:Intended for aquarium use (implied saltwater/reef)
- Ease of Maintenance / Access:Includes stand and updated outlet; designed for accessible maintenance
- Additional Feature:Includes LED light
- Additional Feature:New outlet design
- Additional Feature:Comes with stand
CPR Aquatic AquaFuge 2 Medium Hang-On Refugium
In case you want a compact, ready-to-run hang-on refugium that’s customized for growing macroalgae and hosting copepods, the CPR Aquatic AquaFuge 2 Medium is a top choice. You get the refugium body, a new-style LED, and a hang-on powerhead so it’s ready out of the box. Its unique baffle system keeps substrate and microfauna undisturbed while maintaining flow to avoid dead zones. At 18.5″ x 4.5″ x 12″ and finished in black acrylic/plastic, it mounts behind your aquarium cleanly. Use it to reduce nuisance algae in the display and to cultivate supplemental food for fish and invertebrates.
- Mount Style:Hang-on-back (HOB)
- Primary Purpose:Refugium / macroalgae cultivation / nursery
- Material:Acrylic / plastic
- Includes Pump or Air Drive:Pump included (powerhead included)
- Intended for Freshwater or Saltwater:Intended for aquarium use (reef/marine and general aquaria)
- Ease of Maintenance / Access:Hang-on-back with baffle system to simplify maintenance and reduce disturbance
- Additional Feature:Unique baffle system
- Additional Feature:Medium-size grow area
- Additional Feature:Acrylic + plastic construction
Factors to Consider When Choosing Hang On Back Refugiums
While choosing a hang-on-back refugium you’ll want to match it to your tank’s size and overflow configuration so installation is straightforward. Consider flow rate, lighting needs, and how much media or macroalgae space you require for effective filtration. Also check build materials and durability so the unit lasts without leaching or cracking.
Tank Compatibility
Check your tank’s physical fit before anything else — measure rim thickness and lip clearance, back-to-front depth, and any rear overhangs so the refugium’s hang-on clips, inlet/outlet and footprint will mount securely and stay accessible. Next, confirm the HOB unit’s maximum rim spec (many require ≤12 mm) and compare the refugium’s front-to-back depth—typical HOBs span ~3–6 inches—so inlets and outlets align without strain. Make sure rear-mounted equipment, hoods, or overhangs won’t block installation or maintenance access. Match the refugium’s external dimensions to available space and guarantee it suits freshwater or saltwater setups. Finally, choose models with appropriate features—light access, baffles, sand-bed compatibility—based on whether you’ll grow macroalgae, house copepods, or run a microfauna refugium.
Flow Rate Needs
Aim for a flow that moves water through the refugium at about 5–10× its volume per hour so macroalgae get nutrients without your copepods being flushed out; anything under ~3× risks dead zones, and over ~15× can shred delicate plants and sweep away microfauna. You should also target a residence time of roughly 5–20 minutes per pass so nutrients and grazers interact effectively. Size pumps 10–30% above the calculated need because head pressure and plumbing reduce measured flow at the refugium. Match inflow and outflow geometry to keep water moving several minutes through the macroalgae zone rather than short-circuiting. For dual-chamber or baffled designs, tune flow to preserve gentle circulation in the macroalgae chamber while keeping return flow steady to minimize microbubble transfer.
Lighting Requirements
Flow and residence time set the stage for nutrient exchange, but light determines how effectively macroalgae will scrub those nutrients — so pick a fixture that delivers about 30–50 μmol·m⁻²·s⁻¹ (roughly 0.5–1.5 W/gal) at the algal surface and highlights blue (400–500 nm) and red (620–700 nm) bands to maximize photosynthesis. You’ll want 8–12 hours daily to steady nutrient export without driving blooms or large oxygen swings. Size the fixture to give even PAR across the growbed; multiple low-profile LEDs or a diffuser prevent hot spots and dark zones. Choose adjustable intensity or dimming so you can tune growth and limit nuisance algae. Finally verify the mount and housing clear the HOB design and resist splash and humidity for reliable, long-term performance.
Media And Space
While planning a hang-on-back refugium, match the internal volume and usable tray area to the biomass you want to support—generally at least 10–20% of your display tank’s volume—so macroalgae and copepod populations have enough room to thrive. Choose media layers that fit: shallow sand beds (1–2 in) or grow trays need adequate depth and baffle clearance so flow isn’t blocked. Prioritize horizontal surface area and compartment layout for live rock, mats, or macroalgae stands—more flat area equals larger populations. Design for maintenance: removable trays or accessible chambers cut cleaning and harvesting time. Finally, balance media volume with flow path so residence time allows nutrient uptake without creating stagnant or anoxic pockets; aim for steady, even circulation through each compartment.
Build Material Durability
At any time you pick materials for a hang‑on‑back refugium, prioritize long‑term resistance to salt, light, and mechanical stress so the unit stays clear, rigid, and safe for years. Choose acrylic or polycarbonate bodies — they resist saltwater corrosion and yellowing far better than cheap plastics, keeping clarity and structural integrity for multiple years. Use heavier‑gauge acrylic (typically 3–5 mm or more on small units) and supported edges to minimize flexing and cracking on aquarium rims. Select corrosion‑resistant hardware like 316 stainless or durable plastic fasteners to prevent rust failures. Look for UV‑stabilized materials and marine‑grade adhesives to avoid brittleness and delamination under prolonged light. Prefer integrated baffles and removable trays made from rigid stock; they survive cleaning and flow stresses without warping.