Personally I don't care for 1 1/4" pvc plumbing, my local stores hardly stock any fittings at all for it. 1 1/2" should give you all the flow you need without finding out later your return is restricting your flow. I think above ground pool plumbing is usually 1 1/2" so there is good parts availability.
PVC pipe size | ID | OD | GPM - low pressure | GPH - low pressure |
1/2" | .50-.60" | .85" | 7 gpm | 420 gph |
3/4" | .75-.85" | 1.06" | 11 gpm | 660 gph |
1" | 1.00-1.03" | 1.33" | 16 gpm | 960 gph |
1.25" | 1.25-1.36" | 1.67" | 25 gpm | 1,500 gph |
1.5" | 1.50-1.60" | 1.90" | 35 gpm | 2100 gph |
Pool pumps will achieve a higher pressure than most sump pumps that I am familiar with so they can push more water through a smaller pipe. Pool pumps achieve a considerably higher pressure than what a sump pump can reach.
What are you going to use on the end of the return line inside your tank? Spray bar? Some other type of outlet?
I would recommend a minimum of a 1" return... above 1" is really depends on your system and how much flow you want to achieve. If you are going to restrict the flow at the end of the return line in your tank it doesn't make a lot of sense to run a whole bunch bigger return pipe.
The 2650 max GPH the DP 10000 quotes is really fantasy. That is the flow with zero head and no plumbing attached to the outlet to restrict the flow using special water that they water down to make thinner water. I would bet that with the head and fittings that pump will never really do over 1,500 gph. Every aquarium pump I have ever used is this way, this is not unique to Jebao. On the flip side when you are running your system and you actually do get 1500 gph I think you will discover that is a but load of water and a but load of flow!