Can you use super glue on outside of fish tank?

Common super glue is actually harmless to use on an aquarium, but it might not yield the desired result.

This article describes why super glue is safe:

Super glue is safe for use in fish tanks because it becomes completely inert when moist or wet. The main active ingredient of the glue is Cyanoacrylate. When in contact with water it forms strong and stable bonds. Essentially, it polymerizes in 30 to 45 seconds. Cyanoacrylate is by definition an aquarium safe glue, as water is an accelerant (or catalyst) of the curing process.

The drawback is that superglue is brittle and might crumble with pores and tiny gaps that let water leak anyways.

However, It will be brittle and it will leave small gaps, which does not make it a good sealant.

This exact property results in it being applicable to small areas only.

Though it is an aquarium safe glue for decor, it’s only good for certain types of decor.

To seal a leak you probably have to use aquarium-safe silicone because it stays flexible and can adapt to tiny movements of the side walls without crumbling.