I'm guessing if his Grandad had it, it's original sealant, but that's a good point. Original silicone is harmless, and ok if it does not leak. But if it appears the tank has been resealed at some point, there is always some small chance the wrong sealant was used. Many household silicone based caulks have fungicide added to them, for use in kitchens or bathrooms, and the added chemicals can kill fish as they leach out into water.
If it's solid coloured caulk, replace it, it's either got fungicide or it's acrylic based, also no good for tanks.
If a tank has been resealed it should not be hard to tell, it won't look as neat as a professional job, and there will probably be scratch marks here and there where the scraping was done.
Usually the fungicidal caulks are coloured. White is most common, but there's almond & brown too.
Black, as you see in some modern tanks, is also available but it's usually labeled as exterior only in a hardware type store. Black for tanks usually has to be purchased from an aquarium supply source.. I've never seen it anywhere else so far.
But there may be a clear version of silicone caulk with fungicide in it around too. I have not ever checked to see if the ones with fungicide come in clear, now I think of it.. I only buy them in white, to match the tub.
Edit. You say it was inherited. I hesitate to ask If your grandad is still with you ? If he is, can you ask what he used as a sealant, and maybe when he built it ? If not, it may be best to reseal, simply because there isn't any way to know for sure what was used.
Silicone sealant only hit the market in the 60's, and it was very, VERY expensive when it first came out, but if he was a handy guy, he could have used it. I recall getting my first all glass tank in the early 70's, before that, they were made with metal edges. There are also tanks built of plywood, with a glass viewing panel in front. The plywood is sealed with epoxy or marine paint and then silicone.
As I said, earlier commercial tanks were made using chromed metal 'L' channel & relatively thin glass, compared to a modern all glass tank. The glass was glued into the channel with a black adhesive that I think was asphalt based. Then silicone was used only to seal the inner joints, to prevent leaking and prevent contact with the metal and glue, not to stick glass panels together. That only started after the price came down. If they are intact, those metal channel tanks are fine to use, and can be resealed as well, if need be.
Be nice if you could post a pic of the tank.
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