What happened to one of my hillstream loaches?

I have 5 hillstream loaches in a planted 30 gallon tank, living with 6 rosy barbs and some invertebrates. One of them is way smaller than the rest. At first it was shy and hiding, but eventually became as active as the rest. Then I decided to add some additional gravel to my tank, because my plants kept getting uprooted. I had a lot of hardscape to move around to do this, it was messy, the water got muddy, I had to do a large water change, etc. The 4 bigger loaches had no problem with this, and they are having a blast in the new and improved aquascape. The small one kept hiding for a while after the changes, and I though it was just stressed. But then it stopped hiding, yet it doesn't eat. It just sits here and there, mostly on glass, moving around a bit but not much, and completely ignores any food. It's got visibly skinny. I'm losing hope it will recover. I'm wondering though, is this stress doing all this, or did the stress trigger some disease that it already had? Should I worry about it infecting other fish with something? Kind of hard to believe an animal would just let itself starve to death because it got a little fright?
The water parameters were fine (0, 0, ~7) last time I checked.

 

Could be it wasn't ammonia but something else nasty that doesn't show up on a test kit you stirred up. Is this the SW01? That could explain why the others aren't as effected since the spotted is actually a different (closely related) species, not just a color variant.

 

Could be it wasn't ammonia but something else nasty that doesn't show up on a test kit you stirred up. Is this the SW01? That could explain why the others aren't as effected since the spotted is actually a different (closely related) species, not just a color variant.

No, the spotted one is doing great. It really got used to the company of the others, roams all around the tank, and now the only behavioural difference between it and the lineolatas is that it hardly ever sits on glass. The sick one is the smallest lineolata.

 

No, the spotted one is doing great. It really got used to the company of the others, roams all around the tank, and now the only behavioural difference between it and the lineolatas is that it hardly ever sits on glass. The sick one is the smallest lineolata.

Unfortunately I've got no idea as to why that one would be the only one effected then. Maybe it does have some underlying illness that was made worse by the stress. Maybe quarantine him if you have the ability? Sorry I couldn't help more

 

Unfortunately I've got no idea as to why that one would be the only one effected then. Maybe it does have some underlying illness that was made worse by the stress. Maybe quarantine him if you have the ability? Sorry I couldn't help more

Yeah, I guess the reason of the size difference might have been poor health and not age, as I assumed.
I thought of quarantine, but since my main hypothesis was some exaggerated reaction to stress, quarantine sounded like something that would make it worse. I returned another fish (barb with injured tail) from the quarantine bucket to the main tank yesterday, and I let the spare filter to dry out, so I cannot do that now anyway - I'll be leaving for 3 days tomorrow and won't be able to do frequent water changes an uncycled bucket would require. So now the choices are either leaving it in a tank and hoping it's not infectious, or giving up hope it might recover and euthanizing it.