If it wasn’t like this when you bought it, it’s very likely something banged against it. That is an impact spot and could be problematic.
Now…..
Do you ……have…… to replace it? Probably not
Would it be a good idea to replace it? Probably, but not necessarily
Would replacing it be safer and give you peace of mind? Probably yes
If it were I, I would replace it and turn this one into a reptile tank or sell it off for terrestrial animals. I will admit I am a worry wort when it comes to water filled fish tanks after having had one suddenly burst dumping 20g onto the floor and another develop a leak out of the blue. I am an old goon who enjoys the peace associated with maintaining fish tanks. I do not have peace, am anxious, and am not relaxed when I know there is a potential serious tank issue. Others may very well differ and that’s fine……I’m just sharing my thoughts since you asked.
Watch for the occasional sale on new tanks, buy one, and get ready to relax. I recently had to replace a leaking 65g with a new 55g. It wasn’t a major ground soaking leak, but was enough to warrant action. Water is powerful and exerts decent pressure against the tank glass and a75g is nothing to sneeze at.
Good luck.
Thank you so much for your reply!
While I can easily forget these things consciously for short periods of time, among the daily grind, I do think it adds to my subconscious anxiety and when I shared this information with my husband, about the gouge in the glass, he was immediately very, very worried... and he hasn't even seen it yet. Knowing how upset he would be if it leaked and the possible consequences that could have for my fishkeeping. as we are co-owners of our home, I am actually more concerned about how he would deal with a leak situation than I am about the water. He is not a laid back person. lol
The tank is in my basement so the underfloor is cement, but on top of that we have engineered wood which was put in by the last owners so not in perfect shape. He gets quite unhappy when I accidentally spill water on that and it has warped the engineered wood in places.
All that said, considering my office, fishrooms and my son's building area are all down here, I think your recommendation or "what I would do" is correct. What I would like to ask is: If I add silicon correctly the the outside, will it prevent any possible leak? If the answer is no or not guaranteed, I think I will replace the tank.
The thing that keeps me from rushing out and getting a new 75 gallon is the amount of physical labor involved in swapping out a tank of that size, particularly when the tank is cycled, filled with fish and snail that you are wanting to breed and hopefully sell, concerns about the health of the animals, etc. Being physically disabled, I would need to reach out to my LFS for help. My husband and I can't physically move a 75 gallon. And allowing others to move and help set the new one back up again would make my life that much easier.
Please let me know what you think about using silicon (or other products) to "fix" the tank if that is indeed possible.
I may not have all the experience in the hobby that you do, but I have certainly experienced enough horrible aftermaths of the price of not knowing or making the wrong decision (or having it made for you) that I get what you are saying. I am so done learning things the hard way.
Thanks again!