You can test where it comes from by filling with water and putting a piece of paper under it, then check to see which side of the paper its from. After that, you can seal with aquarium sealant. However, this requires a dry tank and it needs 2 days to dry.
I might go witha new one...
This seems decent!
At about $25 with shipping, this seems great.
If youre willing to spend a bit more, you can get a 5 gallon marineland hex (or other types) for like $40ish bucks, it comes with the filter, too.