It is much better to look at but not like I was there, I can't see any cracks that would be damaging any farther but It looks like about 1/8" deep at least. It being circular like that & on the said pane, I honestly think it will support enough strength to not hurt a thing, also adding a slightly larger piece over it with silicone I, I think may add some security.
Here is the deal, NO one will ever tell you that it isn't going to break because of that divit. This is going to be a personal conscience thing, can you sleep at night knowing that all that water could possibly explode onto your floor?
Find a level spot outside & fill it & let it sit two weeks. Then decide from there if you feel confident about it or not. I can't see enough detail or honestly say any different because It isn't at my house with me making that decision as you will have to. If I had worries about it from my experience, I'd do one of two things, sell it or as I first mentioned, I'd remove the pane & take it to the glass shop & get a new one. It is cheap, & if you take your time cutting it out, & putting it back in then there is no reason to think it would be less stronger than what it was before the divit as in new tank. These tanks are held together by silicone & square cut glass & some sort of frame around the top & bottom, some don't even have that top frame anymore LOL
I still think it will be fine, I'd fill it outside just to be safe but probably would decide rather quickly & be done with it. Now with that said, you can point fingers at me for advising you something that I doubt anyone would want to be responsible for LOL but that is just my honest OPINION, which is just that, wish I could tell you for certain but that just ain't going to happen.