Sizing return lines.

[Randys]

The outlet on my external return pump is 1-1/2" diameter. I plan on using a wye fitting to split the line and feed two nozzles.
Should I keep the 1-1/2" all the way to the nozzles or reduce to 1" at the wye fitting?

 

[Drstrangelove]

I would want it to be a total of 1.5" or more at every point until the water discharges. In other words, if you have 2 nozzles, they would total the equivalent of 1.5" or more. The entrance, interior and exit of the fitting would total 1.5" or more.

Otherwise, it seems to me that you'd have back pressure, which is not a good thing to have with pumps. I'm not well versed, so I'll be looking to see someone else chime in on this, but from a mechanical water flow viewpoint, the overall total pipe diameter should not be reduced imo.

 

[duanes]

It really depends on the kind of fish you keep, and what they might require.
If you use 3 x 1/2" returns you will creat pressure with lots of turbulence, and get lots of surface movement (aeratinn), and directional flow from each. If you have riverine fish this may be optimal.
If your fish prefer calmer water (types such as anabantids or discus) larger pipe diameter like 2 x 1.5" , this will provide a less focused gentler result.
Centrifugal pumps, are made to be throttled back at the outlet end, so the back pressure from reducing the pipe size is not problem.
Because many of my fish were rheophillic and preferred a highly oxygenated strong current environment, I always kept my returns 1/2", and throttled them btack more drastically to provide lots of turbulence as water returned to the tank, as in the photo below.

I would sometimes add venturi valves to returns which concentrate flow even more, and suck air into the discharge, to creat even more turbulence and provide extra aeration