Please, for the love of all that is good...only get two clownfish for your tank...and do NOT attempt any of the species listed above as your first anemone! They are the most difficult anemones to keep in captivity and should only be kept by the most experienced anemone keepers.
180 is too small for multiple pairs of clowns. They are very territorial, and while they may not venture too much farther than 12" from their anemone, their territory stretches MUCH farther than that and they WILL kill to defend it from other clowns. Most report that A. Percula requires at least 100 gallons PER PAIR (two pairs, 200 gallons, etc.) for long term success in captivity.
You do not have to buy them paired, but you could. You could also buy Captive Bred juveniles and they will form a pair in time. It will be faster if one is noticeably larger than the other.
Also, you will NOT be able to rear the fry in your main tank. They will be eaten shortly after hatching if you do not move them to a rearing tank the night that they hatch. It is a lot of work to rear clownfish larvae, so be aware of that as well. Joyce D. Wilkerson's book "Clownfishes" is a great read to get you started on both clownfish in general and also breeding.
