I have two in my tank and they love each other like they've been together for years. I had 2 tanks in my house for 3 years...they are both gone for a bigger one...I have now a tank of 160x50x50 cm (575 liters). In both tanks I had a cleaner wrasse. Because I wanted to make one big tank of the 2 smaller, I put the 2 cleaners together in the new big tank and immediately they liked each other. For 2 days they where busy with a pairing ritual and this morning I found some eggs on the back glass. Maybe I just got lucky with them, I don't know but it's possible to have 2 in one tank...even if the thank is not that big.
I also agree about the cleaner wrasse being left on the reef. The reason they die so quickly in captivity is because in the wild, they eat parasites off mostly big fish like sharks, whales and dolphins. These fish really aren't good to buy.
As a professional aquarist who has been keeping marine systems for over 30 years and has even held the position of Curator of Fishes at a major public aquarium, I'd like to recommend that you DO NOT BUY THIS FISH! I myself have kept this fish under the most exacting conditions, and have even had them spawn for me numerous times. However, I must point out that the two fishes which spawned the most just so happened to be the only two, out of a batch of 16 I acquired at once, to even SURVIVE more than two months. This is too wasteful to be acceptable. Even worse, there are now countless fishes on the reefs today searching in vain to find some cleaners which are no longer on the job due to having been overcollected for the aquarium trade. To add insult to injury, the Cleaner wrasse can only eat the parasites which are on the OUTSIDE of the infested fish. As it happens, the vast majority of parasites are lodged quite deeply under the surface of the skin where the wrasses cannot even reach them. If you want clean, healthy fish, quarantine every specimen and treat it as if it were infested, for it almost certainly is. It astounds me how few marine hobbyists are responsible enough to prevent disease outbreaks in their systems by undertaking a few simple precautions. In my opinion, the use of Cleaner Wrasses & shrimps is a sign of laziness and weakness as well as a near-complete waste of effort. One day, this hobby is going to come under much heavier restriction than it suffers today, and the overcollection and waste of species like the Cleaner Wrasse will undoubtedly be one of the main factors leading to such legislation. Do yourself and the hobby a big favor and avoid such species.
I know that a lot of people buy these when their fish get Ich. Not a good idea because when the Ich is gone your cleaner fish will have nothing to eat. If you have a clean, healthy tank this wrasse will have almost nothing to eat.
My Cleaner Wrasse has lived for several months without any help from parasites on my other fish. In fact, they also groom the fish by eating dead skin from the other fish. They do have voracious appetites, and if you are not dilligent about feeding enough frozen foods to the Wrasse, yes, it may die. Frozen foods have been the only type of food I have had any luck feeding them with. They are incredibly interesting to observe as the other fish fawn all over them to be cleaned. I would recommend one if you are willing monitor its needs.
I would just like to stress that Brian Braden above is 100% correct. Why is it these fish are sold as 'low maintenance' fish? Over 95% of these fish bought die within a month, almost always from either starvation, (refusing to eat) or malnutrition (eating the wrong foods i.e. brine shrimp, which the cleaner wrasse does NOT benefit from.) Surely much better off in the ocean...