Don't get caught up in all the talk about friction loss, fluid dynamics,etc. when you plumb, i can tell you from experience that to much emphasis gets put on minimizing using 90' elbows, this, that, and the other thing,and the advice about it usually doesn't come from someone who has plumbed pvc for 30 years, when you have to plumb something into an existing configuration such as yours, don't worry if there is 2,3,more of whatever to do the job, those compression couplings, and adding union's every 2' to everything, are crutches, and quick ways out of doing a proper plumbing job, but taking the time to hard plumb, like it is from the grey union on the heater return, all the way to the pipe your cutting below the ground, even if there are a few more 90's etc. is still a lot better than seeing unions and cheap gimmicks glued everywhere, now i am not talking about the unions on your salt cell, or the heater, pump,etc, i am talking about the ones you see between pvc pipe that say nothing more than...i am not willing to think this out, and this will work...take your time, approach it like legos or something, dry fit things as you go, take your time!