You need to get Super Glue Silicone on the top where it first started leaking, this stuff is incredibly strong, one of the very few glues that really work in emergency, it works in wet and better in dry, then empty water down to leak and try and get it dry then apply Super Glue silicone right over the seam, use painters tape to keep the lines straight, have tape no longer on than two minutes or you will never get if off, then work your way down top to bottom of seam, apply two applications, and then when you are ready you need to redo manufactures silicone with a good aquarium silicone, and then apply Super Glue Silicone on top of it all over both sides when tank is empty, it's best to redo the entire tank, do two applications. This Super Glue Silicone worked great for me when one of my tanks leaked, see bottle below, you can get it on Amazon