Rigid PVC should be more than enough to prevent dipping...here's my old calfo system:

http://members.shaw.ca/harvwong/10.08.05/DSCF0004.JPG

Pic of what you see in the tank:
http://members.shaw.ca/harvwong/04.08.06/IMG_0176.jpg

I remember that system! What was the longest length you had to cover? Any problems with disturbing the sandbed (or was this tank BB?)?

My tank was BB so no worries about the sandbed. The longest length was 45" with a 14" segment at the end (it was L-shaped). It wasn't a true calfo manifold as it wasn't all one continuous "loop". I split the manifold in half with a SCWD so that only half of the "loop" would fire at a time. Worked quite well, but too bad the SCWD adds so much headloss to a pump. In retrospect, an Ocean Motion 2 way would have been better as you can use much larger pumps with it.

Uh, pardon my ignorance, but do you have to provide an "excess" of flow so that water makes it through to the final outlet at the end of the line (so that you don't have the most pressure coming out the first outlet, then less at the second, even less at the third etc?).

I'm about 90% sure that this is the way I'm going to go. I'm going to try and do an external overflow box so I can maximize the space in the tank. I don't want to go BB, so I'm sure that will be a problem I will have to deal with later.

I want a very random flow, so I guess I'll look into an Ocean Motion. I don't think I want to use a SCWD again, because, like you meantioned, of the headloss.

I guess that makes my next step figuring out what pumps to use, and how I may want to drill my new tank. Should I be looking at doing a few returns through the back of the glass?

Good question...I'm pretty ignorant too about this....anyone?

Couldn't you add ball valves to all the returns, and dial the ones closest to the return more closed, with the ones at the end fully open? Tweak until you get even flow? I've also wondered about this, and it's the only solution I can work my mind around.

Thats what I was thinking as well. However of every photo I've seen, nobody has used ball valves or anything, they all look like Harveys setup.

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No ball ball valves required if you use a pressure rated pump.

Hey Danny, can you explain this? I'm a pressure rated pump dummy :)