Not enough, you have to trace further where the hiss is coming from or further develop your english skills reporting the details. Nothing personal there but it is what it is. For instance, a simple comma placed between the words 'tank' and 'cap' changes everything somewhat. Becomes two questions rather than a run-on incomplete sentence, and where you are really at anyway. One's english or more specifically the non-use or incorrect use of it can kill you here.
If cap, then old ones sometimes leak at hand tight when the gasket hardens and cuts a groove in it. You simply take like a big set of channellocks and then overtighten cap just a bit more. Can run for years more like that if you are not hamfisted.
If crack and above liquid level in tank it's new tank time.