Cory body shape, tummy indentation

[silvercory]

I have just purchased a pair of corydoras wotroi yesterday from my local fish shop. They are quite large there body is about 1.5 inches long not including the tail fin. When I was feeding the fish last night I was sitting on the floor looking up at them and I noticed that they have a smallish indentation on there abdomen / tummy. I was wondering if this is normal? Or if is something to be concerned about?. I have looked at some of my other Cory’s in a different tank and they seam to have a bit of a indentation but not as pronounced as the ones I have just bought. It is difficult to get a good look at the underside of a Cory!! The fish appear to be in good shape apart from this, there sides are not sunken in.

[MatsP]

Corydoras melanistius

is the current official name for C. wotroi, as far as I can tell.

Without a photo, I don't know if it's possible to identify what (if anything) is wrong with your fish.

--
Mats

[silvercory]

I have tried to take some photos but the quality is very poor and you cant make anything out. I will try again but it is difficult to take a photo when they are on the bottom of the tank.

[apistomaster]

Corydoras are fed little if anything from the day they are captured until they reach a local fish shop.
Many have lost body weight which is most noticeable from the pectoral fins to the caudal peduncle. They very often have concave abdomens by the time you buy them. Most LFS do not do much to build up fish they hope to sell within a week so your Corydoras may not have eaten much for 2 to 4 weeks.
They may also be infested with internal parasites which are draining the life energy out of them.

These are some of the many reasons why new purchases should be initially placed in a quarantine tank. It doesn't hurt to medicate them with praziquantel such as Hikari PraziPro and metroniadizole. In such weakened conditions, the Corydoras are not able to compete with other fish for the available food.
If you have access to any small live worms that would be my first choice as the first foods but newly hatched brine shrimp are also very good. An advantage of brine shrimp nauplii is their small size. They may be easier for starved Corys to eat and begin getting their digestive system used to processing food again. If you catfish responds to baby brine shrimp then as it begins to gain some strength and the stomach begins to expand they will be able to eat frozen blood worms or mosquito larvae. It takes as long or longer to restore them to perfect condition as it did for them to get run down.
A healthy Corydoras should at least have a fairly flat abdomen. Once they begin to eat normally their bellies will become noticeably convex instead of concave. It takes a long time for them to rebuild their muscle tissue to normal. For many weeks they have probably been drawing energy from their muscle tissues and normal digestion as a source of energy is bypassed during extreme starvation. To conserve energy the fish tend to be very lethargic and do very little of their normal rooting in the substrate for food. All the armored catfish are difficult to detect signs of starvation because of their armor plating until they are in advanced stages of starvation. This it what makes it so hard to restore them to good condition.
I can't be sure that this is what you are dealing with but what I described is a common condition.
When I can see the Corydoras, and Plecos for that matter, before I buy them I check two things:
1. Is the belly concave?
2. Has the Corydoras lost muscle mass from its body?
If one or both of these visual cues are present I will not buy the fish.
Corydoras showing these signs have a poor prognosis.
Good luck. It is often possible to save some badly starved fish.