The Fluval Hang-On Breeding Box gives you a safe, separate space right on the side of your aquarium to raise fry, protect pregnant livebearers, isolate a sick or bullied fish, or acclimate new arrivals. It hangs on the outside of the tank and uses gentle air-driven circulation to pull in your aquarium's own heated, filtered water, so the temperature and water quality inside always match the main tank. It works on both freshwater and saltwater aquariums and comes in two sizes: a Medium single-chamber box and a Large box that divides into up to three chambers. Because it runs on air, pair it with an air pump and airline from our air pumps and accessories, add a Hikari check valve to protect the pump, and keep Repashy Spawn & Grow on hand to condition breeders and feed developing fry. Ideal for breeding livebearers like guppies, mollies, and platies. Choose Your Size Feature Medium Single Large Multi Chamber Volume 0.3 US Gal (1.1 L) 0.5 US Gal (1.9 L) Chambers Single open chamber Up to 3 divided chambers Water circulation Air-driven Air-driven Mounting Hangs on tank rim (external) Hangs on tank rim (external) Best for One batch of fry or one isolated fish Several batches or separating multiple fish Features Hang-on external design keeps it out of the tank and saves swimming space inside Air-driven circulation constantly exchanges water with the main tank, so temperature and water chemistry stay matched Clear walls make it easy to watch fry and monitor an isolated fish Built-in fry zone lets newborns settle safely away from the current Adjust the air flow to keep the water movement gentle for delicate fry Works on both freshwater and saltwater aquariums Large model includes removable dividers to create up to three separate chambers How It Works The box hooks over the rim of your aquarium on the outside. An air pump (sold separately) feeds air into the lift tube, which draws water up from your tank, circulates it through the box, and returns it. Since the water comes straight from your main aquarium, it stays at the same temperature and quality as the display, which is far safer for fry and stressed fish than a separate, unheated container. Dial the air flow down to keep the current gentle for newborns, or up for stronger turnover. Which Size Is Right for You? Medium Single (0.3 gal): a single livebearer giving birth, one batch of fry, or isolating one small fish. Compact footprint for smaller tanks. Large Multi Chamber (0.5 gal): raise or grow out several fish at once, separate an aggressive individual, or keep different batches of fry apart using the up-to-three-chamber dividers. Recommended Pairings Air pumps and accessories, required to run the box since it is air-driven Hikari Airline Check Valves, keep water from back-siphoning into your air pump Repashy Spawn & Grow Conditioning Gel, conditions breeders and feeds developing fry Freshwater livebearers such as guppies, mollies, and platies, the classic fish to breed in these boxes Fish food and nutrition for growing out young fish FAQ Does it need an air pump? Yes. The box is air-driven, so you will need an air pump and airline tubing, both sold separately. A check valve is strongly recommended to protect the pump. Can I use it in saltwater? Yes. It circulates your tank water and works in both freshwater and marine aquariums. Does it need its own heater? No. It continuously pulls in your aquarium's already-heated water, so the inside stays at tank temperature. What is the difference between the two sizes? The Medium is a single 0.3 gallon chamber. The Large is 0.5 gallon and divides into up to three chambers, so you can separate multiple fish or batches of fry.
