It was fine until the second week of my new tank established.

All of my other Cichlids are fine but Red Kadango.

I have two big male Red Kadangos and they are roughly the same size about 20 ~ 25CM. One of them constantly gets chased and beaten up by the other.

The one get chased, I can see there are a few small bits of scales are missing from the body. The body color is not that shine anymore turns to a bit dark.

The one chasing, boday colors are very nice and no wound on body at all.

Is this because they are use to be in a 6 ft tank and now suddenly change to 5 ft?

They were fine in my 6 ft and fight some times but never like this. Their color were both good.

Any advice? Sell one of them? or leave it until they settle down and fine out who is the boss?....

It is just what happens with cichlids sometimes it just goes to crap In this case it was probably the move into the smaller tank, the fish were probably at a similar level of dominance in the 6ft tank but after the move when they cased each other out and one of them was found lacking.

As for what to do, add more big fish for targets or remove the bullied (or aggressive) one if the fish is getting bullied to the point where it may get killed or is living a crap life.