For easthetic reason you could do this.. But it likely wont hold any sealing propperties. If it is inside and submersed you could try to fill it with TEC7 TRANS. That's an absolute non toxic polymer that cures submersed.. What you could try is take a plastic syringe, fill it with some kit from the top, put the plunger back in put a needle on it and stick this it in the bubble and slowly fill it up. If there aint to much dirt on the glas inside the bubble the kit will mask it beeing there once it is completely filled again with kit. You can even do this without draining the tank.
If the tank aint leaking yet, than it is what it is, keep an eye on it and hope is all you can do. It also all depends a bit on the volume of the tank, tanks up to 100 litres do not hold a vast ammount of pressure and a minimum of strain with temp differnces in relation to surface area. Imagine glass expands when it gets warmer and retracts when it cools again that's a few 0.1 milimeters per meter larger glass surface multiplies this creating more strain on the seals. In winter time with cold nights and warm days with central heating, temps fluctuate in the tank making the glass constantly work and straining the kit. This likely is the cause that some tanks show loos seams after some years of use. Depending on how clean it was and kit quality during constrution.
It likely aint melted, because kit doesn't realy change consistancy at temperatures bellow 180°C. But the glas does, so actualy there is a constant slight friction between glass and kit and streching it. Most heaters submersed can't reach a higher temp than 100°, because water starts boiling at 100°C. Remember that oldschool physics classroom demonstration, boiling water in a paper vessel above a gas burner..
🙂 Heaters getting hotter boil the water and you hear the bubbles pop from the heater element.
As said it the risk all depends a bit on the volume, i still got a 25 litre tank in use for years and it has a chipped off corner at the bottom. It bumped somewhere during transportation and chipped some glas, that's why it got discarded by it's previous owner and i'm stil using it. If it were a 250 litre tank than i would not dare to fill it up again and take it in use.
🙂 Tanks up till 100 litres have a redundant kit job anyway and are relative to it's glas thicknes, volume and weight awfully strong. Going larger it gets less redundant and higher risks and need more precise construction.
