afilter is right on here. Just make sure you know your corals before you buy them. Realize that some LPS
have sweepers that extend out at night, sometimes 6 inches or more to kill others. Also, a lot of leathers give off toxins that will kill other corals placed too close to them, I use carbon on my reef tank with softies for this reason. You really have to do the reasearch, but if done right, you can really cram them into a tank.
A good rule of thumb is to not place LPS
within 6 inches of any other coral. Same for the leathers that give off toxins. SPS
compete with each other mainly by trying to out grow each other, and I don't have but one myself but I hear you can really stuff a lot of them into a tank when they are small. Too bad they demand high $$ lights and cost a ton for the nice ones.