Quote:
Originally Posted by Stitchawl
I floated their containers in the tank for almost three hours to allow the temperatures to sync. Then I lowered the tops and allowed some of the tank water to merge with the container water. After another 30 minutes I let the new fish swim out into the tank on their own. No extra stress.
Stitchawl
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This could be where the build up of toxins is coming from. This is the way I was taught to add fish:
1.Turn the light off in the aquarium to reduce stress.
2.Lift the lid and place the sealed bag containing the fish in the water so it floats.
3.Let the bag float for ten minutes, then open the top of the bag.
4.Put some tank water into the bag.
5.Close the bag and allow it to float for another five minutes.
6.Repeat steps four and five once. For sensitive fish such as neon tetras repeat two or three times.
7.Place the small net into the bag, lift the fish out, and quickly transfer it to the aquarium.
8.Discard the bag of water in the sink (never in the aquarium).
9.Leave the light off for several hours to allow the fish to adjust to the new setting.
Your fish are in the travelling container for maybe 4 hours (including the journey home from where you purchased them), mine no more than 40 minutes. Stressed fish will produce more ammonia and four hours = a lot of ammonia in the travelling water.
Try testing the pH levels of the travelling water and see how it compares to your tank - as little as 0.5 difference is enough to kill sensitive fish like neons.
My other suggestion is Neon Tetra Disease. Have a look at this article and see if it matches what you are experiencing:
Neon Tetra Disease
I hope it is not this disease but the loss of colour and difficulty swimming you mentioned make it a possibility.