Double R

Last weekend I was watching our Clarkii Clowns swimming happily around the tank together. The male clown would quiver in front of the other, which from what I have read is a good thing. But when I looked in the tank later that day he was badly beaten up, his tail is all but gone and his lower fins are shredded. At the back by his tail his scales were all bloodied. Is this normal for the so called mate to attack like this. He has been put in a hospital tank and treated with medicine from the Pet shop. Will his tail and fins get better. Is there anything else we can do to help him get better

The LFS shop in Rotorua has some small blue tangs for $90.00 each

Thanks

Complete newbie here and I'm sure you're gonna think me thick but what size tank would you use a RO/DI unit for, we have an AR620T