Angelfish, discus and South American cichlids live in slow moving blackwater streams. They are more comfortable in tanks that feature gentle water movement rather rapid moving water. Therefore the best aquarium filter for anglefish is one that has a gentle flow. If your angelfish are being pushed by the water flow, it is too fast for them.
Hang on the back (HOB) filters are those that hang on the back of the aquarium. A tube draws water into the reservoir and back into the tank. It passes through the filter media in the reservoir. Commonly this is foam and charcoal. Peat moss or other material may be substituted for the charcoal. These filters provide mechanical, biological and chemical filtration.
If the filter is sized for the tank, the flow is slow enough for anglefish. Some HOB filters have a flow control adjustment. This is a feature to keep in mind when purchasing an HOB filter.
Restricting HOB Flow
If a HOB filter doesn’t have a method to reduce flow, some hobbyists use other methods to reduce flow.
Method 1: Cut the toe off a pair of clean pantyhose and put it over the intake tube. Secure it with a rubber band. This reduces the amount of water entering and exiting the system. This will also prevent small fish from getting sucked into the filter.
Method 2: Another method is to use foam or mesh at the outlet into the tank. This diffuses the water as it goes into the tank.
When modifying, watch that the reservoir doesn’t back up so that the filter overflows.
Power heads are used to increase flow and aeration in the tank. These allow adjusting the water to a higher or lower flow. Air stones in an undergravel filter provide a slow gentle flow good for black water tanks. Power heads can be used on undergravel filters instead of air stones if more flow is desired.
Canister filters sit in a cabinet or on the floor behind the tank. These have adjustable flow rates. Some have adjustable rates to allow higher rates at different times of the day.
Internal filters are submerged in the tank. These filters have adjustments to control water flow. These filters are best in smaller aquariums, or supplement another type of aquarium filter.