Talk:DIY Plumbing 101
From The Aquarium Wiki
Strongly suggest that the pipe and gravity flow table is validated from several sources, including PVC manufacturer specifications & fluid dynamics engineering manuals, and updated with accurate information. The stated flow rates are extremely misleading and are a small fraction of the true, natural circulation flow rate of schedule 200, schedule 40 or schedule 80 PVC and CPVC rigid pipe.
As an example, applying Bernoulli's Equation to standard schedule 40 PVC 3/4" pipe (however actual I.D. is typically .82", not .75") with a 24" drop from the tank to a sump or canister housed under the tank the raw flow rate is:
Gallons per second: 0.29 gps Gallons per minute: 17.7 gpm Gallons per hour: 1066.8 gph In this example, the actual straight gravity flow is 3.2 times GREATER than the table in the article. Caveat: fittings, valves, 45- or 90-degree elbows, filters and UV sterilizers each add varying amounts of "head" back pressure, regardless of pipe diameter.
Following the included information would result in a plumbing system that is severely oversized and inefficient for most aquariums.
Additionally a basic issue that most people do not take into account is that the larger the inner diameter of your return pipe, the greater the volume, and therefore weight, of water your pump must push back up to the tank. Before you add any of the other fitting or filter items, your "head pressure" will increase expontentially going from a 3/4" to 1" pipe simply because that additional .25" results in nearly a 1.5x increase in water volume over the same distance.
Double the pipe I.D. from 3/4" to 1.5" and you increase the volume of water in the pipe by 4x.
Now you need a more expensive pump for higher head created by the extra volume/weight which also consumes 3 - 4 times the electricity to operate. All to support a maximum flow rate that your tank probably doesn't require.
I have a redundant dual-inlet & dual-return system using 3/4" Schedule 40 PVC with a combined real flow rate of 2x764 gph (1,428 gph) with redundant mechanical & bio filters, UV sterilizers and several fittings, valves & elbows. If I pressurize the flow from the tank to a very low 6 psi with power head pumps rather than rely on a gravity siphon, the system will support just short of 3,000 gph flow rate. More than any 150 gallon cichlid tank should ever need.