I got mine because I adopted goldfish, which made a huge mess and smelled so I rehomed them and used their 10 gallon as a tank for bettas. The ten gallon wasn't even supposed to be running, because I broke rules when I bought it, saying I was getting a small tank n.n
Then I put Jazz & Friday in, and my dad was okay because it was only ONE tank. Then I got Sunny, and then Jazz passed :( but now my dad knows I can handle them well and will let me get another tank if we can ever find room for it.
I'd suggest emphasizing the use of dividers and divided tanks so they can compromise.
Hopefully they agree!