Try taking a razor blade , single edge , and cut the silicone from around the edge of the bulkhead and from under the lip of the bulkhead on the inside of the tank . Take your time and it should work loose . Cleanup all the old silicone before you put new bulkheads in . They should come with gaskets so no need for the silicone in the first place .
We are meant to be the stewards of the Earth and her creatures....not the masters.