Cleaning crud behind silicone?

Any chance that there has been some scraping done to remove algea and they have run the scraper under the edge of the silicone? I have several tanks where that is true. Once the silicone is loose at the edge, I've found no way short of replacement that will get it sealed again. New silicone sticks very poorly to old silicone. Once loose crud will collect there.

There is a fix that I have used but it comes with some risk. If it does not go too deep into the silicone, you can take a really nice sharp knife and carefully trim the loose silicone back so that there is not a flap left for the crud. Then you can remove the collected stuff as normal with just a sponge.

DANGER NOTICE--- If you slip and cut it too far, the seal is ruined and theoretically could leak. Make sure you really want to do this before starting.

Cleaning from the front will not get it out if it is behind the silicone.

I am assuming you want to clean it while full. Can't help with that. If it is not occupied, I have done this successfully several times. You will need silicone and a razor blade at the bare minimum. You have to remove the old silicone then use the blade to carefully but thoroughly remove the crud and silicone remnants. Sometimes crud is actually etched into the glass that's when you will need a powder polish like cerium oxide, which you can also use to buff scratches out of a tank. Just make sure you rinse it good and get rid of all the powder before you reseal the tank. Use the buffer wet, nice and wet. If its not etched in, but its hardened like calcium carbonate that is common from saltwater, then EDTA and a toothbrush works great and wont scratch the glass. you can also use this method to get that impossible crud offf of glass tops.

Silicone is best applied in a small bead running down each side which is smoothed by a gloved finger. Look for any bubles or missed spots. it usually leaves a smoother finish if you wait till the first coat has hardened most the way before filling in gaps. If you have more specific questions pm me. I pretty much only buy used tanks, I fix them up before stocking them with fish / shrimp.