How old is the tank because that sounds like a seal leak to me. On old tanks and even sometimes new ones there can be a problem with the seal where a bubble formed in the silicone seal at the factory or any other multitude of things and it doesn't cause any problems for years and out of nowhere it starts to leak because of it. I just recently got a 200 gal tank for $100 because it started leaking on them. Resealing a tank is not that hard to do and there is plenty of great videos to watch and articles to read but here is a brief summary of what I did.
First I went to the hardware store bought a whole box of razor blades to cut out the silicone, you might not need as many as I did but I went through about 30 of them. I also bought my silicone there I don't care if I get yelled at for this but is EXACTLY the same as the aquarium silicone that is a hell of a lot more money, you just have to make sure it is 100% silicone and it is the same thing it will save you at least $50 by using the stuff from the hardware store. Also I would buy a silicone beader it creates a nice uniform shape with the seal which is one of the most important things t have when sealing a tank.
Once I get home I started cutting out the silicone, you want to only take off the old bead and NOT I repeat Not the silicone between the panels as this will just make it a lot harder on yourself. After all of the silicon is out of the tank you need to clean the inside of the tank with acetone because it evaporates quickly and it will kill and remove anything that is still sticking to the glass, most importantly you have to get all of the edges that silicone is getting put on or the silicone will not stick properly to the glass and you will be all the back at square one, again. Now comes the fun, putting the seal back in, I started in left rear corner and put my first bead across the back stopping at each corner to shape the bead correctly with my beader. Once I did the whole bottom I did all of the sides the same way stopping to shape after each pass. After you have sealed the tank you have to wait for the silicone to cure this will take about a week in a warm dry place, two weeks if it is cold, or until the smell of vinegar is gone for about 3 days. Now you are able to test your newly sealed tank and most likely it will hold water again.
Remember that there is no guarantee that this is your problem and that the Internet is your friend with this matter. I just am giving you my experience with leaky tanks and I have had no problems with what I have done and hopefully neither will you.
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