I know this post is mostly tongue-in-cheek, but I'm going to take it very seriously. Primarily because there might be someone in this situation who has a legitimate problem.
Can you afford all of the aquarium supplies you're buying? Are you prone to irresponsible spending? Do you refuse spending on hobbies that are important to him/her?
If you answered yes to any of these questions; stop buying stuff for your aquarium.
If not, then address your relationship. No healthy relationship is maintained with lies. If s/he is controlling, badgering, and generally unpleasant about reasonable expenditures, it's time to discuss why. Couples therapy is amazingly helpful. If his/her behavior doesn't change or s/he's otherwise abusive, it's time to get out of the relationship.
If s/he is just nervous about the cost of the hobby, have a candid conversation about your expectations and his/hers. What are his/her fears? Have him/her explain them. Explain how you will address those fears, if they are reasonable.
Ultimately, you should both agree upon a budget. If you have, for example, 200 dollars a month that is allotted to your personal (not household) use, then how you spend it is your business.
Relationships are cultivated and maintained with mutual respect, kindness and compassion. They should be at least as important to you as your hobbies and, like your hobbies, require upkeep. If that reality is incompatible with your lifestyle, you need to reconsider your marriage.