You guys have me thinking now.
My tank is too small to fit a 2" thick piece of foam across the back, and I think even a corner or even half circle in the center would still be too much.
One of the biggest issues is finding a pump that would only turn over a 7 gallon tank two or three times.
My idea is to run a HOB style filter.
I'll use a pump that puts out 42GPH. Put the pickup at the center of the back wall, then push the output to the bottom of the filter housing. I would split half of the flow to each side. I would make a spray-bar type output that pushes the water down against the bottom of the housing to minimize channeling thru the filter. This would cause the water to flow up from the bottom, thru the filter pad, then out the water fall return back into the tank.
Splitting the output of the pump would put 21GPH (79.5L/Hr) thru each half, 3x turnover for each side.
5 cm/min of flow thru a 2" filter (5cm) comes out to 1 min of contact time for the water to pass thru the filter (typical setup from what I've read thru so far). So, If I were to use a 6 inch (15.25cm) filter hight and stuck with 1 minute to flow thru the filter, this would give me a flow rate of 15.25 cm/min thru the filter. This is within the recommended range, altho a bit on the high side.
So using the equations posted earlier:
Area of the filter = tank turnover rate x volume thru filter x 1000 / (flow rate thru mat x 60)
(6)(79.5L/Hr)(1000cm3/L)/[(15.25cm/min)(60min)] = 521.3 cm2 = 80.75 in2
If each half of the filter was 7.25" long, this would give 14.5" in total filter length.
Area = Length x width, then width = A/L => 80.75 in2 / 14.5" = 5.56" width for the filter.
I'm thinking of putting a 1/4-1/2" thick piece of foam with similar density around the pickup to keep baby shrimp out of the intake (similar to how a center overflow would be set up). This should keep debris out of the filter, which should prevent high flow areas that can occur in a normal setup. So the higher flow rate that I would have shouldn't be an issue.
What are your opinions on this?
I may have a couple of conceptual errors, but it's late and makes perfect sense to me right now. I hope this makes sense, I may have to revise it this afternoon.