spartan said:
How do i match the input and output of the sump so they are the same?
Several options ...
A. Use a float switch in the sump, with a pump that is significantly bigger than you need which goes on and off as required to maintain level - downside risk is that one of those times the float switch will fail or the pump won't start and prime.
B. Use a pump in your sump which is fairly closely matched to the overflow box's flow rate, carefully adjust a throttling valve, let the pump run continuously and hope for the best. Downside risk is that some particles or other variable will change either of the flow rates, leading to a gradual change in your aquarium and sump water levels.
C. My personal favorite, use a sump pump that is a little bit bigger than you need, let the sump pump run continuously, and tee in a float operated bypass valve at the discharge fitting of your sump pump which diverts pumped water directly back to the sump instead of to your tank when the sump level falls. See
http://www.aquaticeco.com/index.cfm/fuseaction/product.detail/iid/8539/cid/2084
With all of the above options, to avoid a flood in the event of a major failure, the extra water capacity (i.e. freeboard above the normal sump operating level) of your sump MUST be enough to hold the extra aquarium tank water present between the normal operating level of your overflow box and the low aquarium tank water level where the overflow box will lose its prime.
Also, with option C and especially with option B, you need to have a fairly good idea of the actual flow rate you'll be getting off of your overflow box in order to choose a properly sized pump. I'd suggest a test with your overflow box installed, a one gallon bucket and a stopwatch - then buy a pump which has say 1.3 - 1.5 x higher flow rating than the flow rate you meaured. With option A. this doesn't really matter much and you can use a pump which has any rating between 1.5 and 3 x your measured flow rate and it will still work fine. And remember when choosing your pump to consult the pump curves for the pump's actual flow rating with the necessary feet of head added between your sump level and your aquarium water level (plus some extra feet of head added to overcome plumbing losses). Check out
http://www.aquaticeco.com/index.cfm/fuseaction/product.detail/iid/8804/cid/4016 and then click the Pump Curve link below the picture of the pump.
Of course there's also cheap and dirty option D. - just hang any good sized pump near the bottom of your sump and let it suck air when the sump level falls. This is actually the lowest risk option in regard to failure, but it will be noisy as hell, and is probably going to trash a pump on you once a year or so because of repeatedly running dry and pump cavitation.
Important PS - in all of the above, you'll also need a full sized check valve in the line which runs from your sump pump discharge to your tank unless you're using a spray header or otherwise discharging sump return water above the water level of your aquarium. Without a check valve, if the outlet of the sump pump return line is below the level of your aquarium water, if and when the sump pump stops this line can and will act as a siphon and could allow however much aquarium water to flow backwards through the sump pump and into your sump until the end of the return line is finally exposed to air to break the siphon. If your aquarium is say a 55 gal and if the freeboard area of your sump holds say 5 gal, you've got a disaster on your hands if say 30-40 gal of aquarium water is allowed to siphon backwards through the return line and sump pump body and overflow your sump. See
http://www.aquaticeco.com/index.cfm/fuseaction/product.detail/iid/8258/cid/2015
~