Talk to me about this please bud

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From what i can see on this side your using this coast to coast for overflow duties?
I see what looks like a cut out section on the right side, is this your only overflow access point?
This we have to work out your rough through-put as you have two different spectrum vaules going there.

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14ft x 1 & 1/5th
2 x 2ft & 3/4 inch for your 2 primary returns on each side. Primary because of less headloss compared to your spray bar returns....(is the plumbing wider with a reducer at the bulkheads?)
another what...3ft for the modular tubing over the top at 3/4 inch.?
Im going to have to work it into metric at some stage cause i have know idea on your guppy measurements. If your top sections total pipe dimensions equates to the diameter before the reducers, then the only factor is straight headloss & you have a ball park vaule. If it doesn't we're going to have to venture further into the equations to negate your 2 values divided by the hight. Im sure we have wizz kids on the site, if not i'll calculated it for you tommorow.