What size is your tank?
When did you add fish after it was cycled? Did you cycle fish-in or fishless?
What is your pH?
They require excellent water conditions. Mature and stable tanks. Like things just right. Even skipping water changes or adding tap water to make up for evaporation can throw off the water enough to make them unhappy.
They like the water a little warmer, 82F is ideal. Your other fish might not. If you raise the temperature make sure you have enough O2
. Warmer the water the harder it is for fish to get O2
How long did it take to cycle this tank and how long has it been since it was cycled?
I would check your hardness, GH
, and also KH
Are these from a local hobbyist or fish store?
Fish can be stressed and subsist in an aquarium sometimes for awhile. Maybe the moving things around stressed him and he was stressed too much. These fish are somewhat delicate. Things you might not think of being a big deal could bee just too much. Then when a fish is stressed, he could be much more likely to become ill.