unwell?

The smallest of my four cories doesn't look well. He's been a bit less active than the others since the move, yesterday I noticed looked rather listless and frequently going to the surface for air. I thought his barbels looked a bit shorter, too. Today the barbels are obviously worn down, the fish looks skinny, holds its tail kind of bent and is swimming aimlessly around in the middle of the water column. I don't know what's wrong with it. The other three look fat and happy, alert, nice long whiskers... maybe this one just never overcame the shock of moving?

In my main tank, I fed the tetras the last of summer caterpillars (frozen) and dropped sinking wafers in a secluded corner for the kuhlis. It worked well- by the time the tetras worked up their nerve to push through plants into the corner, kuhlis had eaten enough their tummies very thick and rounded. Wafers were softened so by the time a tetra grabbed one and dashed off with it, the food was disintegrating so other fish grabbed some as well and the first one didn't choke down an entire wafer. Shrimps also moved in to take some, I always laugh at how once a shrimp is holding a wafer, the other fish don't seem to see it. Would be so easy for them to move in and snatch, but they don't. They only take it away if the shrimp drops the wafer when trying to carry it away.

So I had a good close look at my kuhlis and shrimps, as they were all crowding into the corner against the glass to feed. One of my older amano shrimp and one of the malaya shrimp, their insides look pale and cloudy. I don't think that's good. I've seen this mentioned on the forums before, and it's probably an internal parasite or they are just old and near death. Bummer. I'm not surprised they are two of my oldest shrimp- the amano is over a year old and the malaya I've had six months. Not sure if I should pull them out of the tank, looking up more info to see if it's contagious...

I was watching the kuhlis feed- their little heads pressed together over the food wafer- and there's something alarming about Albert (four years old- not that long for a fish that ought to live to ten) it looks like he has eroded sores on the sides of his head, just behind and above the eye.

I tested water in both tanks. Zero ammonia, but nitrates are a bit higher than usual, about 35ppm. I usually don't worry unless they're over 40... but since two fish look unwell I'll do some extra water changes.