I think most of them are easy, esp. the ones you will find in a shop. Species like furcatus, gertrudae and signifer all make great starts. There are a few that while not difficult take special conditions, like the brackish water one cyanodorsalis. And then there are the tougher ones (at least...
I have keeping blue eyes for many years. 14 species and several different localities of some. Most are easy to keep and breed, but there are a few difficult ones. The preferred water hardness depends on the species. I think most Blue-eyes will last 2 to 3 years and some species will live to...