There's this glue stuff you can use to seal it. I'm not sure what it's called at all because I've never had to use it but I'm sure if you explain your situation to someone at wherever you got they'll be able to help you.
In fact, scratch Home Depot, go to a Fish shop in your area. Most of the shops I know actually make tanks themselves and even if they don't should know how to seal it anyway. I've heard it can take time but it really depends on where exactly the crack is on the tank.
Just lower the water so it doesn't leak and look for some help for that ASAP.