I'd love to speculate;). Sounds like maybe a connection problem between the sump and fuge? If the plumbing between the two doesn't allow for free flow, it would run the sump side dry (assuming you had the fuge upstream from the sump), while leaving a rising level in the fuge (the fuge to sump plumbing becomes a bottleneck). When making a flow through connection like that, I make the add-on plumbing connections a larger area than the ins and out, if possible. What I mean is to calculate the area of the inlet pipe (pi rsquared) and compare to the outlet or connection piping. If you have 1" going into the fuge, come out with the equivalence of 1 1/4" or 1 1/2" pipe. You have to use nominal ID for this.
Of course, but Tee'ing of off the fuge feed line, you can valve off the amount going into the fuge, bypassing to the fuge somewhat and reducing the bottleneck. If the case I have described above is similar to the actual case, this might be an easier fix. It shouldn't drain faster than you can return it if everything is working on gravity with pumped return, so it sounds like there has to be a bottleneck.
