You have a very unusual setup, so I'm not comfortable making a recommendation. The two breakers tied together should be for a 240V outlet, such as an electric stove. The microwave is required to be on it's own independent circuit. So, it sounds like someone modified the wiring and didn't update the labels. That would have me worried.
I will say that I would never run anything other than a nano aquarium on an outlet that shared a circuit with a kitchen. I wouldn't want a toaster or coffee maker tripping off my aquarium. Those can be large loads and a trip would be expected if your aquarium heaters kicked on while making a pot of coffee. Not a good situation.
Ok thanks for the input. Its a newer house (5 years) that I had built so I don't think anything was modified but perhaps mislabeled. Doesn't explain why its tied together though. Guess Ill have to hire an electrician to check out the situation.