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Personally? I wouldn't have them open anything. Quite honestly, the idea of having a bunch of keys in the bottom of a fish tank filled with swamp barracuda seems a bit silly and melodramatic. The fact that they're easily avoided by mage hand suggests to me that he's not really worried about whether they're taken or not.
What I'd do? The keys and barracudas are all window dressing. In the bottom of the tank is a plain, unremarkable rock, buried underneath quite a few of the keys. That's really what he's protecting. It's actually something important and possibly magical altered by a polymorph any object and a magic aura to seem innocuous. Jaster probably doesn't know what that rock actually is, or what it can do - he was just told to protect it by his superiors in the Sczarni, or he'll be the next into the tank.