back in July we got my mother-in-law a 2.5 gallon fishtank with a halfmoon betta and an ADF. that betta died, and so have four others we've tried putting in there, all within a matter of days. at first i thought it might be radiation (her husband had just undergone radiation therapy and was warned to stay away from people and animals) but two died when that wasn't a problem. then i thought it was the food (she had the kind of betta pellets that plump up with water, so i was worried they were choking on them) so we bought her a different kind of food, but the fourth still died. it's not the temperature or water quality, because she's had the same ADF in there the entire time, and he's ridiculously healthy (growing like a weed, always active, nicely colored) it's not an infection or parasite because they never show signs, either in bodily appearance OR behavior, they just appear to be perfectly happy fish until one day they wind up dead. what else could it be?