Acrylic (Plexiglass) should be chemically welded to itself with a solvent made for that specific purpose. Silicone will usually start to leak, as is is not designed to bond to acrylic. A permanent solution would be to go to a glass shop and get a piece of plate glass cut to replace the acrylic divider, then install it with another $5 of silicone. (I'm assuming the tank is glass--If the tank is also acrylic, then buy some Weld-On acrylic cement from a local plastics shop and use that, again, a permanent solution.

How did you get $70 in silicone onto a single divider? Were you using stacks of one-ounce tubes? Even a name-brand aquarium cement should cost less than $15 for a caulking tube, enough to build an entire 100 gallon tank.