I'm not sure about "impossible" to tell apart. Difficult maybe, I don't think I can tell the difference but there is supposedly a difference. Something like males have a more bulldoggish look to their jaw. Basically, since the males are mouthbrooders, their mouths are bigger (and slightly different shape) than females' mouths. I had both females as mine, which also made me wonder if having an unpaired female might lead to them becoming eggbound. It might be better to have a pair, but then you have the problem of them trying to breed. I say "problem" because while it's cool to see fish breed, it's more work on you to raise the fry, and it's hard on the males who don't eat for months at a time while brooding. They don't hardly get their strength back up before having another brood, so very often the males end up starving to death. It's one reason I've sort of steered clear of trying a pair myself.
Mandarins can be trained onto mysis, but the onus is on you to be sure they still get enough as they're sort of ponderous about eating. ("Is this food? Is this food? Is this really food? Oh, I guess it was, because my tankmates just ate it all.") So possible to try one but a little more work for you at feeding time.
Yeah, I love rabbitfish but shied away from suggesting one for two reasons. 1) They're pigs and grow quickly. The max size of most rabbitfish are really too big for most of our tanks. One reason I've been looking for
Siganus doliatus is that it's one of the smaller rabbitfish. It also happens to be one of the prettier rabbitfish, bonus. The only thing is ... well ..I've been trying to buy one now for TWO FREAKING YEARS. I ask and I ask and I ask the stores, and they never get them in.
For some reason they are just not coming into western Canada. I don't know why, because they seem to be readily available south of the border.
You can try the similar
Siganus virgatus though, also very pretty, and although max size is slightly larger than the doliatus, it's not by much.
Here are some pics of my 2 virgatus:
Very fast growers. The little guy I bought in December was barely 1.5" when I got him (or her). Now in March is already half the size of the larger, easily 3"-3.5" I would wager. Great personalities though, total sweethearts.
Oh and I have the same thing with my little guy putting his hands in his tank. He has to be lifted to get up to tank level but he really wants to help with feeding and all that. I don't worry about him and the rabbits though, I mean he'd have to grab the fish or something, and that's just not going to happen, they see us coming and shy away from the surface.