Hello

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Yes, what corrosive said is very true. Most people will tell you to keep at least six, same with the white skirt tetras. I think four is a good number depending on where the tank is, other fish in the tank and how the tank is decorated.
Problem is that if you get another 3 black neons you will be over stocked. It is usually ok to be a bit overstocked as long as you look after the tank very well, and have a very good filter. However i would not reccomend this for a new tank. ESPECIALLY a tank that is apparently still cycling, that could be fatal to all the other fish. Wait a few months and add one black neon at a time every few weeks, if you can afford another small filter or something, i would reccomend that too

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The white skirt tetras should be ok as a pair, as long as the tank is in a quiet place.
I would also reccomend you buy lots of aquatic plants for your tank if you havent already, to make the fish feel more secure and slightly improve the water quality

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EDIT: Whoops, just realised you have blind cave fish in there too

. Your tank is already overstocked(depending on the size of the fish). You should probably take a few fish back to the shop, i know its hard, but if you dont it could effect the health of your other fish, and could eaily kill them in a tank that isnt cycled

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Good luck with whatever you do!