corydora with injured mouth

Question
We have had a 29 gallon tank for 3 and a half months.  The tank has a Whisper 40 filter and an AquaClear 70 filter; the filtration is 500 gal/hour.  About 30% of the water is changed weekly. The water temperature is 78 degrees. When the water was tested today the results showed 0 ammonia, 0 nitrites, and nitrates in the range of 20-40.  Currently in the tank we have a male and female platy, a female guppy, 6 glo-lite tetras, 6 neon tetras, 7 cory cats and 4 baby platies that have evaded the other fish for the past 2 months.  

The problem we're having is with the male emerald cory cat.  It looks as though he has lost the bottom portion of his mouth.  He still has both barbs, but that's it.  The curved bottom part of his mouth is missing.  The area isn't discolored and it doesn't have anything fuzzy or filmy on it; it's just gone.  He's active and is eating as much as he ever did and is plenty social with the other cory cats.

From what I have been able to find online, having the wrong gravel can harm cory cats.  The gravel in our tank is a mixture of small and medium sized smooth river rocks.  I'm not sure if our cory hurt himself on gravel or in some other way.

What I would like to know is what steps we can take to make our corys as safe and healthy as possible. If this happened because of human error, we want to correct the problem ASAP.

Also, now that this has happened, what should be done to aid our injured fish?  Will the missing part of his mouth regrow and if not, can he have a happy fish life with a defective mouth?

Thank you in advance for your help with this.  

Answer
Hello Michele!

There are two things that can destroy a cories sensitive mouth, nitrates, and, like you said, sharp gravel.

I would start doing more water changes. 25% twice a week is the minimum. If it is 40 at the top it could be as high as 100 low to the ground in a dead spot (which is unfortunately where cories like to rest).

It could however like you said, be the gravel. Here's a quick test. Take a big handful of gravel and squeeze it. If it doesn't hurt you it won't hurt the fish and vice-versa.

It may be able to grow back in the proper conditions if you up your water changes to the previously mentioned amount.

-Nick