J jacob_s please dont listen to this. Every statement is false I dont think even one is truth all the way through.Red congo puffers are awesome they are bright red agressive and predatory and get to about 6 inches.
Black ghost knife fish
Only usually get 10 inches in captivity enjoy planted aquariums
Silver datnoids only get 12 inches and would be fine in a 55 gallon
Short body red tail catfish 12 nches max awesome fish to keep.
Payara are very predatory and get 12 inches in captivity
Red bellied pirhana max size 12 inches insanely agredsive and will eat anything that comes into the tanj thats not a pirhana
Bgk way bigger then that in captivity I can go get a video of one that 18" this weekend.
No 12" fish should ever be in a 55g.
I'll give him the puffer statement but sounds like you want more action then a ambush predator. Most of the time they are not red either.
How about exodons? Another good choice I saw above was apistos. Or you could do a small group of angels and let a pair develop and remove the rest stock it with a group of smaller tetras for dithers, some cories, some easy plants like Val and smaller swords or crypts( not biotype but easy).
As
But I think you could also do a couple electric blue acara and some deeper body tetras like Columbian or Buenos Aires.



