Sick Cory?

[FranciscoB]

They like groups alright, the more the merrier, providing the space is sufficient.
However, I noticed a couple of things in the video, which you may want to consider. One of them potentially has to do with the apparently odd behavior of the single cory. The other has more to do with overall long-term cory happiness and health. I will start with the latter.
The gravel yo have should better be replaced with sand. What you have, allthough relatively fine as gravels go, is too coarse and can be damaging to your cories in the long term, specifically affecting their foraging, damaging their whiskers and affecting overall behavior.
But one issue you may want to check is that it appears that the individual cory showing the odd behavior, is missing an eye, its left eye. If so, that could explain in part the odd behavior. Take a good look; I may be wrong, but that is something I think I saw, perhaps best in the images below (
first, the left side, possible damaged or missing eye; second, the right side having an eye, but potentially showing some cloudiness?). I know sometimes pictures lie (show "oddities" not corresponding to reality, but it is worth a good peek).

 

[nigelnorris]

Yes Francisco, I wondered about the eye thing too. it looks that way in the picture I put up at the top. He's settling down so fingers crossed. They've been in the tank for close to a fortnight now and he's not starved to death yet so is functioning at at least a basic level.

The gravel in view is only part of the substrate, and I've plants going in to carpet most of it it. About 1/4 of the tank is fine white sand, that's where I've been putting all of their food so far, hopefully they'll get the hang of that.

 

[Byron]

Too harshly worded so I've edited down to this.

All of the trials were done in 10 litre tanks. Amongst other species 10 angel fish in one 10L tank. There must be many stresses caused by the tank size alone that don't apply to our home aquariums. Nothing coming from that could possibly relate even to my small 140l tank.

Secondly, as I said above, all they did was compare 1, 2, 5, and 10 shoal sizes. And the more the merrier, no surprise. but that doesn't mean that ten is in any way ideal, it's just the biggest number they chose.

Valid points but I doubt this invalidates the results. I don't see how three and five fish will be any more stressed due to the tank size itself than would ten crowded in? And the ten clearly had less stress. The results are what they are, and numbers matter.

 

[Byron]

Yes Francisco, I wondered about the eye thing too. it looks that way in the picture I put up at the top. He's settling down so fingers crossed. They've been in the tank for close to a fortnight now and he's not starved to death yet so is functioning at at least a basic level.

The gravel in view is only part of the substrate, and I've plants going in to carpet most of it it. About 1/4 of the tank is fine white sand, that's where I've been putting all of their food so far, hopefully they'll get the hang of that.

The primary issue with gravel is the bacteria. Barbel loss in cories is primarily due to bacteria in the substrate, caused by the large grain size, rather than roughness though obviously this too is a consideration.

 

[nigelnorris]

I'm keeping a very close eye on cleanliness, there's some mould showing where brown java moss is dying back so I've changed about 25-30% three times in ten days to get at it. Each time I take out the stuff with moss attached and clean / sterilise it thoroughly, the gravel and sand get vacuumed. Not much algae appearing so I think I have the lighting about right so far.