Adding more fish to quarantine tank

[Madreefer]

A 3" tang in a 20g long? I won't comment on that one. I'm sure there's someone on here that will try ripping you a new one for that alone.
I'm not a fan of quarantine tanks. IMO it causes more stress. That's a different topic all together. But plugging more fish in there is kind of defeating the purpose of quarantining anyways isn't it? Just prolonging your quarantine time. No treatments nor filter is more of a holding tank isn't it?