Ok so I'm a pipefitter. Been one for over 14 years. You are not going to like what I have to say.

Tear it all out now and do it right. Period. Don't screw around and take chances. If you can't do it yourself ask for help. You cannot properly patch a bad PVC joint. PVC CEMENT is not a GLUE. PVC cement works to melt down a thin layer of both the fitting and the pipe. When you push them together you mush all the bits together. It is actually a solvent. As the chemicals melt the PVC the solvents quickly evaporate creating an almost instantly usable bond. Translation: you are mixing and transferring the plastic from each piece creating a molecular bond. Again this is not glue. Glue fills in voids and creates the bond itself. Silicone on PVC is the absolute most horrible crap I've ever seen in the hobby. It just screams look at the guy who either failed to do any research at all or the guy that does a sloppy job and has little pride. Silicone does not properly adhere to PVC. It will fail. Not when either as it will be in a very short time. I've seen tanks piped in using only silicone, GOD it looks horrible. Salt creep and stalagmites everywhere.

But hey it's your tank right? Do it however you feel comfortable...