okay.. now where shall i drain out the excess water?? i hear that i should drill a hole on the overflow section of the tank and make a pipe stick up to the point where water is excess but i dont want to drill a hole at the back of the tank's overflow..
You want to drain the water from the lowest point of the system, IE the last compartment of your sump where the pump is, and IMO it makes sense to drip the good water in as far away from there as possible, so right by your return to the tank. Alternately, you could have a pump on a timer set to remove water from the system and a float valve to replace the water that is removed (jcardona1 had a set up like that if you search for it). Both systems have their pros and cons, with a drain in the sump you can just put your hose into the main tank if you want to change more water and it is never going to flood, and with the pump/float valve system you know your sump will never run dry and I prefer the idea of having a small daily change (say 5-10%) rather than a constant dribble.

