I recently moved my 11 inch comet (7 years old) and his two 6 inch Shubunkin tankmates (both 5 years old) to a larger 45 gallon tank. A couple weeks later I added 2 little (1.5 inch) black moors and 2 orandas (same size) to the tank and somehow within a week of putting them in there with my much larger fish, the little guys killed the big comet by constantly nipping at his body and now they are shredding the shubunkins' tail to pieces. I've never seen goldfish be this aggressive especially against fish 5 times larger than they are. It's so bizarre!
Do shorter tail species just have a natural habit of attacking longer tail species no matter how much bigger the other fish are?

I had a baby black moor once and it was an extremely slow swimmer. Same with Orandas. Im surprised they can even catch up to the comet. You sure thats what caused it? was it over night? fancy goldfish arent normally agressive I dont think.

__________________
55gallon
Light - 6500K CFL, 23wattsx5 5hr/day
Substrate - Red Mexican Clay, Sphagnum Peat moss, Eco-Complete
Misc- Pressurized CO2, EI Dose KNO3, KH2PO4, CSM-B
Plants -Limnophila Aromatica, Stargrass, Dwarf Hair Grass, Christmas Moss, Baby Tears (HM), Dwarf Baby Tears(HC), Rotala Rotundifolia, Ludwigia Glandulosa, Riccia Fluitans

It wasn't overnight, it took a whole week for this to happen. The moors and orandas aren't fast swimmers but it's just that everytime the big fish would swim near them, they'd start pecking away. I'm just stunned by their brazeness given how much smaller they were to be pecking at a much larger fish.