Glowlight Tetra's are sick

I’ve had 5 Glowlight Tetras since November 2005. They’ve always been healthy. No problems. Then I noticed that one of the Glowlights was swimming funny. It was struggling to keep itself upright. The next day it was in the same condition. A few hours later I looked in and saw that it’s whole tail had been bitten off by the other fish. I had to put it out of it’s misery. Two days later I found a second Glowlight in the same condition. This fish has been laying on it's side for 4 days now. I know it's alive because I can see it breathing and it moves it's fins. Neither of them were showing any signs of sickness before this happened. They looked and acted healthy. The tank has been set up since November 2005. It’s a 29 gallon community tank with Tetras, Dwarf Gouramis, Minnows and Danios. I change about 35% of the water once a week. All of my other fish are fine. What could be happening to these fish?

 

What are your readings for ammonia, nitrIte, and nitrAte?

-Chelle

 

They are all at a safe level

 

What are the actual numbers?

-Chelle

 

Ammonia: 0 Nitrite: 0 Nitrate: 40

 

NitrAtes are a tad on the high side (I suggest keeping them below 20) but not absolutely horrible.

Are the glowlights losing any of their color (especially sections of the orange stripe)? Any bent spines?

-Chelle

 

The remaining 3 Glow Lights aren't losing any color and seem fine. I took someone's advice and decided to euthanize the fish using Clove Oil and tank water. It had been laying on it's side for 4 days. All of my other fish seem fine. We'll see how it goes. I think part of the Nitrate problem is from overfeeding, so I cut down on that.