are the harlequins expensive and hard to get?
In Australia they are because you can't bring them into the country. When they do become available here they sell for a couple of hundred dollars each. They are probably much cheaper and more readily available in the US & UK.
is it like chocolate chip stars etc they eat?
They generally eat Linkia species of starfish, blue linkia and red linkia. Some will have a go at other starfish that are similar to the linkias.
what size of tank would u need to have for just a pair of harlequins?
100litres plus for a pr. They get to about 4inches long and need really good water quality. The bigger the tank, the more stabile the water will be.
why don’t they eat anything else but starfish?
They have evolved to hunt starfish. They are a natural starfish predator and by controlling the starfish, they help other organisms survive the starfish's predacious appetite.
how often would u need to feed them?
They need feeding every day. Usually you add a starfish or leg and leave it until it has been eaten. Then add another one.