My larger pump is a Seltz L40, rated at 2.5m and 2800L/H (note, not at the same time
) thay seem to be fairly rare under that name (google search found 20) but they are sold under different brands, mine was called a resun sp-6800.
link
some flow tests I've done with it, divide L/H by 3.76 for GPH
1)raw pump, factory 5/8" fitting, 1m 5/8" hose,50cm head; 1800L/H
2)raw pump, custom 1/2" fitting, 1m 1/2" hose,50cm head; 1136L/H
3)raw pump, custom 2x 1/2"+ 1x 1/4" fitting, 1mx2 1/2"+ 1m 1/4" hose,50cm head; 2050L/H
(3)flow of dual 1/2" only; 1850L/H
(3)flow of 1/4" only; 200L/H
(3)flow of 1 of the 1/2" with 1/4" blocked; 980L/H
4)raw pump, custom 1/2" fitting, 2x 1/2 waterblocks in series, 1m hose,50cm head;730L/H
5)raw pump, custom 2x 1/2" fitting, 2x 1/2 waterblocks in paralell, 2x 1m hose,5cm head;1620L/H
(5) flow per block; 810L/H
note, the 2 water blocks used (both DIY) are high flow, each with a similar flow resistance as 2 right angle copper elbows.
"raw pump" means just the pump sitting in the water, no fitting on the inlet.
pic of my "modified" pump with the custom tri-outlet and factory 5/8'' fitting for comparason.