If you have a leak, then you need to scrape all I mean all of the silicone out of the tank. Use razor blades and perhaps get a parent to help you. Then buy some silicone sealer from a hardware store that comes in a caulking tube. GEI silicone is a good choice which is fish safe and it can be found at most hardware stores. Write it down and take it with you and ask someone for help. Then run a bead of the silicone over all seams in the tank and smooth it out with the back side of a plastic spoon. Then wait a couple of days minimum for it to dry and do a test fill outside. As for the plastic rim, you can clean it really well and silicone it back on as well. Use tape to hold it on and in place while the silicone dries. The key to getting a good seal is to have all of the old silicone off and the tank clean before applying the new. I like to wipe the tank out with some rubbing alcohol to remove any dirt or oil residue before applying the new silicone.