Flow Rates
Introduction
Flow rates within the aquarium are generally divided into two areas, flow within the display tank or circulation and flow through the sump or filter. Flow rates are a given as a volume of water per time, most commonly an hour, written as L/h (liters per hour). Flow rates are also described as turn over rates which are in reference to a display tanks volume written as 30X or 30X's which means 30 times the display tank volume per hour.
Circulation
Circulation is a term typically used to describe water movement or flow within a reef aquarium. Corals in their natural habitat are accustom to quite high flow rates. Put an imaginary box around any piece of natural reef and you will find the flow rate in and out of this box would be much higher than most aquariums. This sort of flow rate suits the life functions of the corals, aiding in diffusion of food and nutrients into the coral and to clear waste out of and away from the corals.
Within the aquarium high flow rates help keep suspended organic matter (detritus) suspended instead of it just settling to the bottom. If organic matter is usable to the corals it increases the useful contact time that a coral can use it and it keeps suspended matter suspended so it goes over the weir and is filtered out rather than settling to the bottom where it will become a nutrient source for nuisance algae.
Flow rates of 25 - 30X times the display tank volume could be considered as a minimum. Many hobbyists will consider 30 times as a very conservative flow rate opting for levels as high as 70X or more. The amount of water flow required will depend primarily upon the organism the hobbyist wishes to keep.
Filtration
Flow though the sump is very different, you want to slow the flow to aid filtration. Increase contact time for microbial mediation of ammonia and other nutrients. Allowing detritus to settle and any micro bubbles to float. Keeping the flow rate lower than the skimmers process rate will ensure all water is processed by the skimmer.
Pumping water up any height is an inefficient process due to the weight and viscosity of the water. A pump rated at 2000L would be lucky to have an actual flow rate into the tank of 1500L but this could be as low as 1000-1200L in some cases. Closed loop pumps will be affected by a loss in flow rates but since the suction side of the pump is connected to the tank it will be much less than the return pump. Power heads and propeller will not be affected by the weight of the water.
How do I provide adequate water movement?
See pumps.
Resources
- Water flow is more important for corals than light. Part 1. Introduction to Gas Exchange by Jake Adams - Advanced Aquarist's Online Magazine
References
FAQ
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.
How much flow should go through my sump?
The water flow through a sump is directly related to the return pump. It is generally accepted that 3-5X the display volume is an acceptable flow rate for the sump and return pump.
