Were they a mated pair or just two fish he put in at the same time? That's what I'm wondering:
Most people seem to agree that just throwing two in the same tank will provoke a war between the two. However, the site says "in their natural environment, they can be seen singly, in pairs, and in large 30+ groups" which brings me to believe that the said pairs naturally came together as a mated pair, thus no fights whereas two random neon gobies will destroy each other.
You're right though. It could depend entirely on the fish but no one's had a good experience with introducing two at the same time yet
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