A covered sump would allow you to top-up to the sump and not the main tank. The overflow(s) maintains the water level in the tank, and the water amount change would show up in the sump.
MY TANK: Planted 10g; 2 x 10W CFL; Fluval U2 internal filter; MGOCPM/black sand cap
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started April 18, 2012
Last edited by Django; 12-09-2014 at 12:23 PM.