Hi bros,
If i may suggest, you may want to try using epoxy. It won't be as pretty as using silicone since epoxy is colored. It will also be harder to scrap it off later if you should for whatever reasons decide to do that. It also will require several mins to harder. But epoxy will stay in position under water and if you put some pressure on it using your finger during the hardening minutes, it should dry eventually to stop the leak in your sump. Just my 2 cents worth but hope it helps.