Did you try a search in this forum or read the FAQ yet? If not you might want to try that, since that's a fairly common question, and the answers you'll find might be better than what I'm about to say. But here's my attempt at a short answer: haddoni tentacles tend to be short little nubs, gigantea tentacles tend to be longer (think of the difference of "carpet" and "shag carpet"). When a gigantea is stressed it might pull in its tentacles and it can be then hard to distinguish from a haddoni. Another difference is the presence of verrucae. Haddoni's have verrucae but they tend to be small, subtle or even invisible at a glance. Whereas gigantea's have obvious verrucae, sometimes even a constrasting colour. (Verrucae are the little "bumps" on the side of the pedal disk, which are used to help hold the anemone in place.) There are other differences too but that should give you a start. When you see enough pictures of both, it will eventually become fairly apparent. Hope this helps.