Thanks for your reply fishfreek,
Okay ... i am only a novice but this is what i think ....
I think the problem you specified will only happen when the sump is below the main tank. But if the sump is above then that problem wont happen. For eg. lets assume that the pump failed, then all that will happen is that there wont b any flow from the sump to the tank. This is because the sump wont have any excess water at any given time. Also, in my case when the pump is off the only water which will run the reverse way (to main tank) is the water in the "line" which is hardly measurable. To clarify here is the link of the image http://img.villagephotos.com/p/2004-...sumpDesign.gif
an image of the proposed design.
Although, i will be loosing a sweet little benefit(side effect) of having the sump below which is that of having a constant level of water in the main tank despite of evaporation.
I think having a sump above the main tank has an extra safety(no overflow ever/autoshutoff) than having it below the main tank ... well at the cost of loosing "Constant water level & reducing the oily film in main tank. What do you think?
~ viv ~