Broken Daystar Airbag Cradle/Cup

[Low_Sky]

I like the whiffle ball idea. I think they should not be able to move inside the pipe to be effective.

Whiffle balls are a good idea if you can anchor them inside the tank. Otherwise, fbenross is right, you'd need to stuff the whole thing full of whiffle balls so they can't just move with the water. You could also cut baffles out of something like 1/8" HDPE sheet (same OD as the pipe) and secure them inside the PVC fittings when you glue up the joints.

[Ducky's Dad]

I got the wiffle ball idea from big rig tank trucks. They use a steroidal version of wiffle balls to control surge in their big tanks, and as far as I can tell, they just dump them in loose. I never noticed a surge problem with the old tank, but that could be because there are so many fittings with internal passages and ridges to impede flow. The new tank will be just a simple, straight pipe with a cap on one end and a drain on the other, and a filler neck on top. I already a case of new balls, but never felt the need to use them in the old tank.