im gonna say stress.. im no expert and have not had very many rays but to me the ones ive had any kind of tankmates in with at that size didt do well. it seems to me when they are real small like that you almost need to keep them by them selves or with a ray or two of simaler size. and to me it seems you need to do smaller water changes more often as a larger one to me seems to send an already stressed ray over the edge. but like i said im not an expert and dont have alot of experiance but with the rays i have and have lost it seem fitting.
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