The problem with the internet is that tank size is basically opinion, and anyone that "wants" a large fish will try to justify cramming it in a a "too small" tank because that just "hafta" have it, not considering territoriality, water quality or other determining important factors.
When in the states I had at least 20 tanks going.
I got "most" 100+ gallon plus tanks cheap (@ $100) or for free, because so many people get fish, and because they don't accept the responsibility of their actual needs end up killing them, or with a smelly dirty tank, and just want to get it out of their house. I've picked up 55 gallon tanks on the curb.
Being a cichlid guy, I feel a 50, or 55 gal tank is only large enough for dwarf cichlids, if cichlids are wanted.
If you have hard water, then something like Amatitlania nanoluteus from Central America, or smallish rift lake Africans.
If your tap water is soft, and mineral poor, smaller Amazonian types, like Rams or Apistogramma, or western African dawrfs like Kribensus.