Wow, that's odd. I don't think I've ever seen bubbles form in the sealant like that before.
It's definitely a weak spot in the sealant but what you want to do about it is up to you.
1. Replace the sealant - Time consuming process but it will definitely remedy the problem. It's not terribly difficult to do.
2. Replace the tank - This is what I would probably do, but I have 4 extra 10g tanks sitting around my house unused atm. 10g tanks are cheap and easy to find so it might be easier.
3. Ignore it - You would likely be safe for quite a long time. Any leak that does happen will very likely be tiny at first and "should" be slow enough to remedy. It's still somewhat risky to do though.