Why is my neon tetra hiding?

Question

I have a 10 gallon tank with 4 small black mollies (I received them when they were babies) and 4 neon tetras. 1 of the neons, the biggest one, stays under the filter and his mouth keeps opening and closing. Any ideas what this can be and how I can help him/her? The other three tend to chase each other around.

Answer 1

Do you have a bubbler to get oxygen into the water?    Either that or that fish thinks you're ugly.

Answer 2

He might have Tetra Disease, quarantine him until he dies or gets better.