But it is more likely that something happened about the time you removed the other fish. Maybe the net you used to remove the two goldfish was contaminated, or you used something to scoop out some water, and it was contaminated. In this way you may have done something that affected your fish. On the other hand, I have noticed many times that fish become skittish, when too many tank mates disappear at one time. Maybe this is a natural fear.
Most fish seem more comfortable in a big group of tank mates.
I would recommend you put the extra fantail back in the aquarium and see if the other fish's behavior changes. I am looking for unusual colored beta fish. Jan S. Reply. Hello Jan, thank you for your compliment. We have Yellow Betta males and females.