Yes it is in my opinion. I believe a tank that size is only big enough for dwarf cichlids.
A single 5" fish is too large for a 20g tank? Lol
I've used 10g tanks for breeding them and 20 gallon tanks. The 20s offer more than enough room.
Going by this logic then the a 4ft long 55g tank shouldn't be used to house an oscar and a arowana would be well cramped in a 12ft long aquarium.
A 20g long is over 5 times the adult length of a convict!