Um, Gen, not to get too personal here (meaning, of course, that that's exactly what is about to happen) but maybe you should sit down with your wife and have a discussion about this issue before you dash off and buy more tanks. Like that she should not buy additional fish without discussion and mutual agreement and all that.
If this has been going on for more than a year it is not likely to change unless you bring it out in the open and talk it over. Otherwise just imagine what that 55 is going to look like in a few months or even days, if she goes on with the practice of bringing home all sorts of random fish because it struck her fancy. What sounds like impulse control and dominance issues at this point can easily escalate to serious anger which is not good for any relationship.
Been there, done that. And wound up with separate tanks and one is His and one is Mine and neither of us is allowed to interfere with the other's management of same. We just decided we would feel so very very stupid standing in front of a judge saying and having him ask "What are the grounds for this divorce?" and having to answer "This fishtank, your honor...."