Neon Tetra Tank Issues

[Goldfish345]

My neon tetra tank seems to be having some issues. It’s a 10 gallon tank with 8 neon tetras. One of the neon tetras has gotten very fat, like it looks like it might burst. The neon tetra has a straight blue line, and has been like this for a few weeks so I don’t think it’s going to lay eggs. I also have one neon tetra that is seemingly stuck in the corner face down. A few days ago he was swimming around face down, but now he is completely in the corner. I don’t think I am over feeding them, and I haven’t feed them for a few days and it’s not getting any better. Any ideas? I noticed the tank was like this when I got back from vacation.

 

[Aiken Drum]

Please take a read through the "unhealthy fish" sticky and give as much detail as you are able.

https://www.aquariumadvice.com/forums/f17/before-posting-about-unhealthy-fish-read-this-32451.html

 

[Goldfish345]

Ok, so I just tested the water and the nitrites were high. I am not sure why. The filter is a double sponge filter (no idea gph). I added prime and some QuickStart.

 

[Aiken Drum]

Please go through the sticky and answer as many of the questions as you can in as much detail as you can.