I had 4 lovely Julii (false Julii I think) Cory Cats in my 60l gravel bottomed tank for about a year and they were great. Very lively and entertaining but all friendly! One died and the 3 remaining ones became a bit more subdued. I recently bought a 125l tank and put nice sand in it specially for the Corys. I bought 3 more False Julii Corys which looked identicle, however when I got them home the new ones were slightly bigger than my existing ones but exactly the same marking pattern and shape. The 2 groups were bought from different LFS
After allowing the 3 new corys to settle in I transfered my origional 3 to the new big tank. Almost immediately fighting broke out and it has been going on for 2 months. Often 2 fish will seem to pick on another, sometimes two seem to be getting alonf nicely then a 3rd or even another pair come slong and they all start barging and chasing. Sometimes one fish (hard to tell if it is alwsys the same on) gets trapped into a gap or corner by the others, when they eventually let it go, usually because I startle them by sproaching the tank, the fish lpoks stunned and just sits for a while. I have tried to video the behaviour but it is not the most aggressive example and I had to do it from a distance on zoom so it is very bad quality but here it is...
Should I be worried or even try to seperate my origional 3 back to the old tank? They seem to have equalised in size a bit now and its hard to seperate them now but I could probably tell.