Circulation is a term typically used to describe water movement or flow within a reef aquarium. It is of vital importance to the health of a reef system and is often undervalued by the hobbyist. Water flow allows various organisms (including corals) to capture prey, remove waste, reproduce, respire and photosynthesise. Different organisms require different flow rates.
How much flow?
The amount of water flow required will depend primarily upon the organism the hobbyist wishes to keep.
Aquarists typically used the term "turnover rate" to describe the amount of water movement in their tanks. A turnover rate of 10x means ten times the tank volume moved per hour, so if the aquarium held 100-litres the required flow rate would be 1000 litres moved every hour.
In general, the bare minimum turnover rate is 10x tank volume for in-tank circulation. The recommended turnover rate is typically 20x with 30x and over classified as high flow. Note this is a gross generalisation, it is far better for the hobbyist to look at the specific organisms before deciding on how much flow is enough.
How do I provide adequate water movement?
- Water flow is more important for corals than light. Part 1. Introduction to Gas Exchange by Jake Adams - Advanced Aquarist's Online Magazine
I want to keep Acropora, how much flow do I need?
As I guide stony coral with short, thick branches are usually sourced from areas of high water flow while those with long, thin branches are found under low flow conditions.
What about my soft corals, how much flow do they need?
Corals with fleshy polyps or long tentacles (e.g. the Euphyllia spp.) tend to come from lower flow.