Hi I found a very nice circuit to maintain water lavel between two preset limits.
I made it and it works cool.
Its require a few component so very chip and all parts are available
Here is the link http://electronicdesign.com/Articles/Index.cfm?ArticleID=9137
Please read the page very carefully including "Reader Comment" .
There u get some modification for the schematic.
Here u get the pages and right schematichttp://rapidshare.com/files/119097670/water_level.rar.html