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Is their a benefit in having it coast to coast on a tank ?

Surface skimming

Here's my crazy idea what do you think ?
http://i1287.photobucket.com/albums/...psedc6599e.jpg

Too much wasted space doing an internal coast to coast. You are going to wish you had that space for live stock. I wish I did a coast to coast external now.

What if underneath the ramp was open ?!to the display tank or would that be silly kinda like a cave

I would stick with a normal corner overflow, the space is more valuable than the surface skimming

How do people make coast to coast work ?

Externally. You could do something like this if you really want a coast to coast.
http://i708.photobucket.com/albums/w...rflow-plan.jpg

But have it flow into a corner over flow.

Did you just wip that up ? Ha ha wish I knew how to use a program like that

Nah I just stole it lol.
Actually google sketchup is free and easy to use.