Ive got 3 of the bigger ones, biggest is around 4 inches. and yep you will never see them- unless you catch them by surprise with the room lights. Actually thats not entirely true, 1 of mine I can see all the time, but only cus the mouth of his cave faces the front of the tank... I didnt actually get them myself, my dad got them around 3 years ago and he was wondering why all his neons and guppies were vanishing overnight!!! So i ended up with them. They are cool fish though, just dont really see them. mine each have their own cave, dont really socailise amongst themselves that i can tell. They eat pretty much anything, the one with the visible cave will eat anything that ends up in his cave- or anything i direct in there. If the lights are on though he wont come out for it, even if it is less than an inch outside the entrance