I have a red oscar about 5" and I bought a veil tail oscar to add to the tank. When I put it in the red ran it down and laid on top of it. When the Veil Tail was able to get away the red would do it again. I just thought they were deciding who was the boss. The next morning the Veil Tail looked awful. I removed it to another tank. Is this normal? Should I keep them seperated or try it again?

welcome to the site.

There really isn't any difference between introducing another fish, or getting another dog or cat. In both cases you can never be sure whether they're going to get on when they first meet. Obviously it's a little more tricky with fish because they are not able to get away from each other if they are in the same tank.

It's probably due to the fact that the red Oscar was there before the Veil tail and he's taken umbrage to this, "how dare another Oscar invade my space"

The problem with veil tail Oscars is they are very vulnerable to damage because of their long fins. So if this aggression is going to be an ongoing occurrence, the Oscar is going to look rather tatty most of the time.

Sometimes rearranging the tank can help with aggression but it's a long shot and probably won't make any difference.

I think you're just going to have to try and give them a little time. Maybe you could put a divider in the tank so they can see each other, but not actually have any physical contact. Again, this could be fruitless, but it may be worth a try.

I'm afraid there's no easy solution to this, it's very very common for this to happen so you're not alone

Some oscars get very 'tank proud' and wont let another oscar in the tank. The best thing to do is to put a divider in the tank and wait for a couple of weeks. Then put the aggressive fish into the others side. Leave a couple of days and remove the divider. If the aggresive oscar still injures the other then just accept they wont get on and rehome one