My excuses for getting new fish:
a. "I was so depressed that I went to the fish store and bought this pathetic-looking, malnurished, sick little betta. If I hadn't bought him, he would have died a horrific, painful, gruesome death." --> That one worked reeeeeally well.
b. "My favorite female guppy just died and her boyfriend is sooooo depressed without her... oh well. I need to get another female guppy to keep him company." --> That one worked well too, but I got some funny looks.
c. (Conversation between mom and I)
Mom: "What are those new little fish in your ten gallon? I told you no more fish!"
Me: "Well, I wasn't planning on getting any, but they were in with a bunch of adult fish and if I didn't take them, the other fish would have eaten them, or they would have starved to death. Plus, the owner gave them to me for free."
Mom: *Irritated, you are too smart for your own good look*
Me: *Cheesy, I'm an angel child look*
d. Nothing... at this point I have so many fish in the 10g that no one ever notices when more magically *appear* in the tank. Hehehe...