best way to add a second clown

[douglam]

so ive had a true perc in my tank for about 3 months. its a 12 gallon tank. i really want to add a second clown since i found a store that sells onyx's. what is the best way to go about this to keep the first one from killing the new one?

 

[Denver aquarist]

Add one that is smaller then the one in your tank. You may see the one in your tank go after the new clown at first but the new one will submit to the bigger one.