Paidey, if you want to keep fish from jumping through that opening there is thing called gutter guard. It can be purchased at Home Depot of Lowes or probably most hardware stores. It can be cut to fit and slides on the wall between the back chambers and the display and fills in that gap so fish cannot jump through it.
Like this
http://www.homedepot.com/h_d1/N-5yc1v/R-100520851/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053&superSkuId=202938895