Corydoras missing fin...

Question
Hi, i hope you can help me with my question.
im a bit worried about a couple of bronze corys of mine, they belonged to my brother and were doing really well in his cold water tank with goldfish and a few others, but he lost interest and gave them to me. i have a number of fish tanks and different fish and decided i was in a good possition to take the corys on. they have been in my standard 2ft, fresh water tropical tank for about five months now, but in the last month or so i noticed they have no "dorsal" fin and the area where the fin should be is almost white. the spike that stands at the front of the fin is gone too, they only have a stub left.
the corys are fat and healthy, active, eating well and seem to get along fine with the other fish.
these are - 10 tetras, 5 other corys (one wild cory, four others whoes sp. i can not be sure of, they are a matalic green colour), 3 bristle noses, a pair of Kribensis, a sucker fish and a few snails.
tho the corys with missing fins look and behave normal, i have noticed they have taken to resting on plants above the gravle, i have never seen the other corys do this.
the other fish are all doing very well, im hoping to fave a batch of young kribs soon.
the pH is...bugger, its quite low...
Amonia is 0.
the pH has been how it is for quite a ong time, and all the other corys are fine, so im not sure if it is the pH that removed their fins.
hope you might have some ideas.
Jessica

Answer
Hi Jessica,
It is hard to tell without seeing it, or knowing how long it took for the fin to disappear. It is quite possible that your Cichlids or even the pleco is picking on your cory. Kribs, while usually rather peaceful, are still Cichlids and may be aggressive towards smaller fish.
This may not happen when you are around, but it is always a possibility and you should not rule it out.
If you don't already, start adding aquarium salt to your tank to help prevent infections and consider moving your corys to a different tank.