The Recommended Flow for Saltwater Aquariums

The flow of a saltwater aquarium depends on the type of aquarium you have and what you plan to keep in it. Fish and seahorses need the lowest amount of flow, while stoney corals need the greatest. The type of flow in the aquarium is an important factor, as you don't want direct jets of water blowing onto the corals or fish.

  1. Flow Through the Filtration System

    • There are two basic types of flow in a saltwater aquarium: flow through the filtration system and water movement in the aquarium. There are many different types of filtration for saltwater aquariums and the system you use will determine your type of flow. Many saltwater aquarists prefer a sump with a refugium (a place where fish cannot enter) and a deep sand bed. A sump is a separate unit of water storage connected to the tank; it provides a location for the refugium, deep sand bed and other filtration devices. The flow through the filtration system should be slow enough to allow for the filtration systems to work.

    Fish-Only In-Tank Flow

    • A fish-only aquarium, also called a fish-only with live rock (FOWLR) aquarium, needs water flow mostly for aeration and filtration. Fish can force water through their gills, so they do not need water flowing toward them all the time. However, without proper flow, debris will build up on the bottom of the tank and slime (composed of oils and algae) will accumulate on the surface of the water. Because of this, the flow through the filtration system is often enough for the fish. However, if you have a heavily stocked aquarium with larger fish, it may be wise to have a powerhead pointed at an angle toward the surface of the water. This will help disturb the water's surface, increasing its surface area, and aiding the exchange of gases between the water and the air. Another powerhead angled toward the bottom of the aquarium will help prevent buildup of debris.

    In-Tank Flow for Soft Corals

    • Corals do not have gills like fish but do need oxygen to survive. Corals absorb oxygen from the water flowing past them, and therefore the demands for the flow in the tank are greater when corals are included. Soft corals require less flow than stoney corals. In general, the flow in the aquarium should be 10 to 20 times the tank volume per hour for soft corals. For example, if you have a 50 gallon aquarium, aim for between 500 and 1,000 gallons-per-hour (gph) of water flow. The filtration system will likely not provide all this flow, so you will need pumps, powerheads, and wavemakers to increase the flow in the tank.

    In-Tank Flow for Stoney Corals

    • Stoney corals need the greatest flow, up to 50 times the aquarium size in gallons, although flows as low as 30 times can work as well. This means that for a 50 gallon tank, you will need up to 2,500 gph of flow in the aquarium. Most pumps and powerheads come with a gph rating; simply add up all the ratings to find your flow (be sure to include the flow through the filtration system).

    Laminar and Chaotic Flow

    • Having lots of water moving around in the aquarium is a good thing, but there is a proper way to construct the flow. Laminar flow is water moving constantly in one direction, similar to a river. The outlet of a pump or powerhead produces laminar flow. Chaotic flow is water moving in random and constantly changing directions. Generally, chaotic flow is more desirable than laminar flow in a saltwater aquarium. Chaotic flows help coral grow in a more natural formation, help stir up debris and better mimic the natural conditions on a reef. To create chaotic flow, use wavemakers (pumps that turn on and off), deflectors for the pump outlets, or simply point the outlets of two or more powerheads at each other.

    The Exception: Seahorses

    • A seahorse aquarium is the exception to the rule of high-flow. Seahorses are not powerful swimmers like fish and cannot anchor themselves to rocks like corals. Therefore, the flow in a seahorse aquarium should be much less than even that of a fish-only aquarium. Usually, the flow through the filtration system is plenty, but you may want a powerhead on a timer to blow through the rocks and decorations to remove debris for a few minutes every day. Laminar flow is particularly undesirable in a seahorse aquarium, as it will blow them around in the water. Use plenty of deflectors, and point the outlets of the pumps up toward the surface of the water.