Hello. New forum member here.
So I have set up a sort of playground tank for my five Glo Danio's. It is a long five gallon Fluval SPec. They were and are always zipping all around, but recently, the pink one (Mr Pink) stopped eating. He just swims casually around on the bottom half of the tank, but will not come up during feeding time, nor does he catch any falling food during feeding time. He has become very thin and when I look at them with a light behind him, I can see that his stomach is totally empty, so it is not like he's bottom feeding scraps when I'm not looking. The other four are healthy and thriving, but Mr Pink is very small and thin now. This started about a week ago after I did a routine 50% water change and rearranged the decor. I do not see any signs of bacterial or fungal infection, and no visible parasites that I can detect. Water temp hovers around 80. As of a couple days ago, parameters were all normal, tested with API. I use filtered water with Seachem Prime. The tank has a few live plants mixed in among plastic decorations. I have tried three types of flake food - Glo Fish flakes, Tetra color flakes, and my normal Cobalt Tropical flakes, and then tried to crush up some Betta pellets, but nothing enticed him. I have also tried soaking the food in aquarium garlic concentrate from Kent, which got the attention of everyone EXCEPT Mr. Pink. He does not seem skittish or stressed, and still gets along fine with the other Danio when they are near, so it isn't like he's being picked on. I thought to maybe isolate him and try using a general cure, but I really do not want to medicate a fish before I know what is or could be wrong, nor do I want to stress him out by moving him into a tiny hospital tank. I really have no idea what it could be.
You can see in the images that the other Danio's are nearly twice the width of Mr. Pink, who can be seen at the bottom, watching me take the photos. I have been keeping fish for years but these guys are my first venture into the Danio world, as well as my first "glo fish", so I am treading in new waters here. I have had them for about months now. Any advice would be appreciated.