my 125g stock (may seem a bit over popuated, but things are working out fine) most important is the filtering though.
1x silver ck 1x gold ck both about 12 in.
2 x sailfin plecos 5 in ea.
1x synodotis feathfin cat 6 in.
3x tinfoil barbs 3 in ea.
The Clown knife is a pretty mellow fish, but can be a little territorial (something the syn cat found out pretty fast lol) but as far as tank mates go; i found that silver dollars/large angels/tinfoils and jack dempseys work pretty nice. Plecos/ smaler catfish/ bichers (considering they're about half the size of the knife or larger. i've seen them go well with oscars, but in a 125 that's pushing it a little.