[WaterLife]

There has always been discussions on substrate-sifting bottom dweller having their barbels erode. Some saying sharp substrate WILL erode the barbels while others say they never have had problems with barbels being torn up by sharp substrates. It's actually the barbels get cut, exposing a open wound for infection to get in and erode the barbels.

In all my experience with bottom dwellers, never have I had barbels errosion because of sharp substrate. No damage at all to the barbels and I have had them live for years on VERY sharp tough rocks (inert rock chips/fragments with sharp edges all around) as well as all sorts of various gravel.

Logically it does make sense to get injured by sharp objects, but I, as well as countless other have kept Corys on sharp substrate and never had a problem. Never a cut or any redness at all, still had very long and perfect barbels. No gill irritation either.

If a fish did get a open wound (from any injury, anywhere on body, even barbels, a fish generally can recover just fin with no further damage (erosion, fungus) unless there is a problem with water quality (and/or general health of fish).

But I have never used nutrient-packed/based substrate so I can't comment as to whether the substrate grows certain organisms that can get in a infect barbel wounds easier.

However I do now keep all my fish on soft rounded sand, just because they are much more happier on it (more sticking face in substrate sifting sand through their gills) and always advise keeping them on sand just for their preference, but experience still tells me sharp substrate is not exactly the culprit to barbel erosion.

For a fix, water changes to keep water cleaner, maybe clean your mechanical filtration more to get the collected gunk out of the water. And as mentioned you can add at least an inch layer of smooth sand over the current substrate just in case the substrate is cutting the barbels.

Do not change the substrate while the fish are in the tank. It can be done, but it's a whole lot safer to remove fish first. But you can still keep the substrate and just add a cap layer (what I would do).

Get some meds to help with the barbel erosion (infection), like the all-natural herbal Kordon Rid Fungus (treats more than fungus). Melafix is safe and should help. There are other meds as well, but I like to stick with the less harmful, natural stuff. Not entirely sure if the erosion is bacterial or fungal though. Haven't heard of flukes being the cause, but I guess it's possible (although I've only heard of them being on the body or gills), I believe those would be "parasites/worms". I do doubt it's flukes though, so I would treat for bacterial or fungal (some meds treat partly both).

I have accidently bought some Corydoras (I believe wild caught as well) that already had barbel erosion. I was worried at first as others have said barbel erosion is contagious, but I put them in with other Corys and no other Corys (or any other fish for that matter) had any barbel deterriation. The ones that had erosion never had a progressed erosion.

There were some with practically NO barbels (just itty bitty stumps) and all of them are still alive and fine (eating very well at that) today. The barbels don't really grow back, if they do it is extremely slow, but even without them, they can live perfectly happy lives, finding food and digging in sand just fine.


EDIT: Just read Rinfish latest comment,
As with humans and any other living creature, a healthy diet does keep immune systems (general fish health) in top shape having them less susceptible to disease and recover that much quicker.
New Life Spectrum food line is usually one of the best you can get. Kensfish brand of food, usually has fairly good ingredients at good prices. Hikari I would advise against based on ingredients and price. The other cheap brands are usually not of quality ingredients.

 

[WaterLife]

It is usually said you can go 3" inch substrate without developing anaerobic pockets.
Some say anaerobic pockets are potential hazards, while some say they are harmless as the gases instantaneously oxidize rendering them harmless when the pockets are released. And it is said the anaerobic bacteria that develop in the anaerobic substrate denitrify Nitrates into harmless nitrogen gas. Theres always been toss up as to anaerobic substrates are good or bad, but so far, to my knowledge no one has a set in stone answer.

If you are worried about the anaerobic pockets, it's best to go with sand that has uniform graded sizes, so all the sand pieces are the same size so they don't compact as much so they allow more water/oxygen to flow through the substrate to keep it from turning to anaerobic conditions. Just look at the sand pieces to see if they look all graded to the same size. Pool filter sand is known to be graded.

But since you are only using a thin layer, any sand should do (not as much sand so water/oxygen can still flow through the thin layer)

Hikari is not that bad, but ingredient wise, it just equates to the cheaper brands using low quality ingredients, but with a higher price tag.
Omega One has good ingredients, but there is some concern with how they list their ingredients, wet weight instead of dry weight, so the order of ingredients in the list can't be trusted, so it can actually contain only a little of the -wet weight- listed actual good ingredients. I still use Omega One, but I only buy if the price is good.
Repashy gel foods are good too. Paradigm has some good ingredients, but not common to find. New Era is ok, but pricey, making it not worth it IMO. The other brands are rather lower quality ingredients, but most of Kensfish has cheap prices on good food ingredients.

I can't say the cheap foods will make your fish sick or weak, but it's understandable to see how better quality food makes a healthier, stronger, quicker recovering fish.

The 1mm are a bit larger than I like, but if you have full grown adults you could get it. Smaller fish can still pick at it though. I just prefer the .5mm as they can take in the whole pellet and chew it in their mouth for a little while. .5mm is a little buoyant so some might float (which I like for my top feeders), but they will slowly sink shortly or a stronger current can knock the pellets down. If you don't like it to float, you can either throw the pellets into the water (the momentum helps them break the water surface easier) or just release the pinch of food under the water surface or just bump the floating pieces into the water. Either size are fine, I just prefer .5mm.

 

[WaterLife]

I won't be as hard on you as others (although the fishes lives are in the keepers hands) as we all make mistakes.

That does look like a severe case of mouth fungus and needs to be treated ASAP. The fish may or may not be too far gone to help, but still try to help him.
I would consider salt treatments, Melafix, Pimafix as well as the Indian almond leaves, as milder treatments which would be sufficient for milder cases. But seeing as this case is severe, a stronger med is needed.
If you tried the milder treatments, they are probably not strong enough at this point and will have little to no effect. So you would need to resort to using a stronger med, but they are more harsh on the fish as well, and the fish may be too weak and die, but it's the only chance the fish has.
Not sure if you can, but maybe half dosages (wouldn't recommend if it's a antibiotic) of the stronger meds can work. There may be some natural potent herbal medicine out there (not sure for gram negative bacteria) that are much less harmful to the fish and still a strong treatment on the bacteria (not as strong as the harsher meds, but stronger than the milder ones)
When using meds, it's advised to provide additional aeration (surface agitation).
Whichever you go with, treatment needs to be done ASAP to save the fish, so don't delay.

Water changes will help keep things cleaner, but won't have the fish recover by itself with the disease progressed as far as it has.

Here is a website on mouth fungus (a form of Columnaris)
http://www.fishchannel.com/fish-health/aquarium-fish-mouth-fungus.aspx

Not an expert on this disease, but I hear kanamycin (active ingredient in med) is effective and usually safer (Don't do half dosages with anitbiotics or they won't be effective at all, do the full strength and course)
Erythromycin is mentioned sometimes, but it does not treat gram negative bacteria, so it won't be effective on columnaris.

With the amount of damage already done, even after treatment the wound will be quite large and still prone to reinfection until it heals more (the fish may forever be disfigured, but can fully recover). So make sure to keep the fish in pristine water conditions to give it the best chance of recovery and less chance of reinfection.

I am not sure if this disease is contagious. If it is, I would treat the whole tank just to make sure all traces are gone. And putting already sick/weak fish into a new environment such as a hospital tank overloads the fish with stress and chances are even slimmer of recovering, but sometimes is necessary if the disease is too risky (dangerous) of having spread to other fish.