I had a problem with the larger of the two filters on my big "fish only" cichlid tank and all of the sudden had a few fish die. One, then a week later another. I didn't know why. Then suddenly, two more fish are under attack and looking like they might die. I changed 50% of the water, which I also tested -- 0 ammonia, 0 nitrites, 20 ppm nitrates -- nothing bad there.
I had parts on order for the filter for the last two weeks and when it started leaking also, I called the manufacturer and we worked out what the problem was, then got the filter working much better than it had been in the 2 months I've had it.
As soon as that filter was working, the fish stopped attacking each other.
Now, 2 days later the torn up tails are starting to heal, I only hope the torn off scales grow back soon and the scars color up from the biting. Aggression levels are back to normal for cichlids.
So, the point of this long story is that even if you test the water and it SAYS it is OK, there are still things that may be building up that are causing the fish to want to eliminate some of their competition. To them, a dirty tank means that there are more fish than there is room for -- so someone must die.
It is not normal for clowns to attack.
Clean your filter, clean your gravel, do a large water change. I'll bet they will behave normally after that.