Here's some info that may prove helpful in your selection of a pump in your build/revamp.
First, determine the GPH of pump you need for 4-6x turnover rate of the display aquarium. Some would say to include the volume of the sump, refugium, etc, in the calculation but in actuality, you don't as including this will increase the turnover rate in the display aquarium. Live rock displacement…save yourself the headache of determining that number.
For example, if you have a 100gal aquarium, you want a pump that delivers 400-600GPH.
Next, determine the head height by measuring the top of the pump to the top of the aquarium, if plumbed into a bulkhead on the aquarium or the highest point of the return line if the plumbing is going over the rim.
Having those two numbers, compare to the flow chart that the manufacturer provides of the pump(s) that you are interested in. Please note, the values they provide are measured under conditions of the pump pushing water straight up, usually with the same pipe diameter as the exit port, ie. ¾" pipe with a pump output diameter of ¾". Any fittings, 45's, 90's, etc, will add to the head height.
Head Loss Calculator
Onto the pumps…there are many brands, configurations and price points that will make your head spin on what to choose. This can be narrowed down from the application and flow requirement for your particular system:
• Submersible (requiring less than 1500GPH @X-ft)
• External (requiring greater than 1500GPH @X-ft)
• Magnetically coupled, ie Iwaki, PanWorld, Blue Line
• Direct drive, ie ReeFlo, Dolphin
For submersible pumps, my preferences are the following in no particular order:
• Eheim Universal and Compact
• Sicce Syncra Silent, the 4.0 and 5.0 are a bit hummy
• Fluval Sea SP series
External pumps I prefer magnetically coupled vs direct drive pumps. The latter is prone to leaky seals at the back of the pump head but for GPH demands over 1500GPH, direct drive is significantly quieter.
Know the "pump style" whether it is flow biased or pressure biased. The difference generally comes down to the spin rate (RPM) of the motor and/or impeller configuration. Pressure biased pumps generally have a higher RPM making it noisier from the rear mounted cooling fan.
If you have an external Iwaki, PanWorld or BlueLine that is out of warranty, you can do a heat shink and fan mod if you find it particularly noisy. Not my mod but a pic I had downloaded from somewhere (probably RC) a number of years ago: