The easy way is to use pvc overflows and cut it where you want the water level.
If you already have a hob overflow, get some scraps of acrylic and extend the boxes so that they are lower in the aquarium. Cut the mounting brackets off of both boxes and glue equal lengths of acrylic to each so that the boxes are lowered the same amount.
If you only need to lower the water level a small amount, you can shorten the vertical tube in the rear box and drill a series of holes in the front box.
