I know i am way too late to help.... but in the future if you ever need to break a silicone bond, use methyl hydrate. just cut what you can and then get some in the crack of the plastic rim. Let it sit for a while and the silicone will let go, eventually. In this situation you would need to have the tank upside down though. We use methyl hydrate at work to clean up silicone and to remove an undermounted sink, if we ever have to...
People alway underestimate how strong silicone is. When we put the sinks in we almost always get... "is silicone all that hold the sink on?!?!?" there was one sink that me and another guy were trying to get unatatched and we were both standing in the sink bouncing and it never let go

combined wheight almost 400lbs