Well, it depends. If you owe a lot on cc's and really shouldn't be buying anything else then she is just looking out for you. Why don't you both sit down and work out a budget together, then agree on an amount you're allowed to spend on your tank each month.
Other than that I would say try hard not to be impulsive, since you know you need to save money, only spend on items you really want. Or set up a reward system, i.e.: I put $300 into savings this month so I can spend $50 on a coral. Something like that, so its not so hard for you to change your spending habits.
It also helps to keep track of how much you spend on everything from food, entertainment, utilities, etc, etc that way you realize how much money is going out. Every time my husband and I sit down and go through all of our expenses we always find areas where we could be spending less money; whether we need to eat in more often or find cheaper forms of entertainment, etc.
Also, keep in mind that you got yourself into this mess and just because your wife might be "smarter" with finances, its also a big responsibility for her to take on and the more you try to change your bad spending habits and help her out, the easier it will be.
Good luck!