I think the whole idea when plumbing a tank that has dual returns is just plumb with the least amount of restrictions as possible. If you are going to add manifolds then you might need to go with a larger pump to achieve the flow you want.
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Where can i fing a pvc wye?
I didnt spend a lot of time looking but i found this. https://www.hy-techroofdrains.com/water-flow-through-a-pipe. looks like 3/4 can carry about 35% of what 1 1/4 can. So 2 3/4 returns could handle about 70%. Still as I said earlier a pressure rated pump would hold up better in this scenario. It would be nice if tank builders started using bigger returns for dc pumps.So for the most part unless you are custom ordering a tank with a greater than 3/4 return hole. Aren’t you almost always going to have to reduce. Most reef ready or drilled tanks I’ve seen are 3/4”. My 100g is 3/4” drilled for a return. I looked for a good pump and nearly all are bigger than 3/4”. I’m using a cor 20 plumbed at 1.25 the. Reduced about half way.[/QUOTE
I didnt spend a lot of time looking but i found this. https://www.hy-techroofdrains.com/water-flow-through-a-pipe. looks like 3/4 can carry about 35% of what 1 1/4 can. So 2 3/4 returns could handle about 70%. Still as I said earlier a pressure rated pump would hold up better in this scenario. It would be nice if tank builders started using bigger returns for dc pumps.
Try this calculator instead..
http://www.freecalc.com/fric.htm
Plug in pipe-size, flow rate, specific gravity, temperature, total plumbing length and then select the appropriate fittings as best you can. It'll poop out a friction loss number to add on to your basic vertical head loss. Use the vertical head loss+friction loss to look up the predicted flow rate on your pump's flow chart.

