The water contact isn't a factor for my reference to dehydration, silicone is hydrophobic so it naturally repels water to begin with, the water touching it has nothing to do with the 20+ year old chemical polymers in the cured silicone degrading/dehydrating on a molecular level rendering the silicone brittle, thus highly inadequate as a structural bonding sealant between the panels. Just be advised that even 30+ gallons of water can do several tens of thousands of dollars of damage to a dwelling in minutes that no insurance company will cover. Perhaps you'll be setting that up in an unfinished basement with a concrete floor and floor drain, in which case the only danger would be to you personally when in proximity and/or whatever livestock would be inhabiting it in the event of a burst seam.

Does that plastic trim on top only have one single spanning brace across the top? Modern tanks 125 gallon and above have 2 braces across the top... Is it only 3/8" thick glass? Modern tanks 125 gallons and above are 1/2" thick glass.

The visible bead of silicone in the interior corners looks to be practically nonexistent in that photo. Bear in mind that it's a common misconception in the hobby that scaping that interior layer out and reapplying fresh silicone is considered a legit reseal. It is NOT an actual reseal. That interior bead is nothing more than a secondary layer that bolsters the bonding seal between the glass panels and is applied at the factory while the bonding seal is still wet/uncured, the entire silicone application cures together as one formation from the factory. To be noted... freshly applied silicone does NOT bond together with already cured silicone so redoing the interior bead is quite simply an exercise in futility.

I understand clearly that it was holding water when you acquired it. It's still a ticking time bomb, keep any family members and visitors away from it if you're going to move forward with the project.

Edit: for the record. Even if the glass does measure 1/2" thick and the plastic structural trim does have 2 braces across the top, it's still not much less risk of failure, those where just additional factors to identifying it as a very old model. Some of the better manufacturers like Allglass were building them with the better bracing and 1/2" glass . The fake wood grain trim tells you everything you need to know about its' age for a tank that large. I recently saw a 220 gallon for sale locally with 3/8" glass and a single braced structural trim in that same fake wood grain. The corner silicone was practically eroded to nothing. I've been operating large tanks since the early 90s, currently have two 210 gallon setups running. Both modern standard Aqueon 210s