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I have five Corydoras concolor, three of which I just spotted have lost their main, long barbels (rictal barbels). They still have their little barbels (mental barbels).

The tank currently just has danios and the corys in it. I don't have specific water quality details, but I am sure my nitrate levels are too high because I am also having a problem with green thread algae. I was already intending to do a full tank clean-out because one of my danios had TB (I know, I'm having a bad couple of months). I have only been waiting to do the tank clean-out until the anti-algae treatment had finished. The corys otherwise appear to be fine. They are lively and eating well. I probably only spotted the missing barbels because I have been watching all of the fish like a hawk for any signs of TB! (none infected, by the way, for over a month).

So what I need to know is:
1)My current substrate is round, smooth gravel, 2-3mm in diameter. Is this too big? Should I switch to sand? Would the danios be okay with that?
2)When the corys are moved to the "perfect" tank set-up, will the barbels grow back? If not, will they be in pain or unable to eat? Should they be euthanised or can they live comfortably?

I would appreciate any help. What a rotten couple of months...

The substrate you currently have sounds fine. The big problem here would be the high nitrates you think are present. Water changes would help a lot. They will reduce nitrate levels and probably also whatever nutrients the algae is feeding on. Frankly, I'm not a fan of any algae-killing chemicals because they just don't work often enough. I don't know what kind of success you're having with your treatment, but I'm just not convinced that they work much.

In most cases, worn Corydoras barbels do not grow back much if at all. This is not a big problem however, as the fish can still thrive. The barbels mainly function to help the catfish find food, but this is not much of a problem in the typical home aquarium. Your cories should adjust to life without barbels provided the nitrates/pollutants are reduced.

How often are water changes normally done on your tank? It could help a lot if you simply did them more frequently.

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And when he gets to Heaven, to Saint Peter he will tell: "One more Marine reporting, Sir! I've served my time in Hell."

Fine sands and gravels need to be disturbed periodically (every few days) to ensure no build ups of static detritus are allowed to occur. During their foraging the corys will be in the front line of exposure to ammonia produced by decomposition in the substrate,and bacteria that can cause decomposition and even tissue rot, so maybe its worth giving the gravel a good going over with a gravel vac.Some people assume that corys eliminate the need for disturbing the gravel/sand, but actually it often increases the need to do it.

Make sure that you keep the substrate clean & do regular water changes.

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Member of the Malta Aquarist Society - 1970.
http://www.maltaaquarist.com