Ulcer on my Malawi Cichlid

Question
Hi Ben, I just noticed tonight a good sized ulcer around the anus of my 3 inch Labeotropheus Trewavasae.  None of my other fish are showing any signs of distress, and beside the ulcer my Labeo is acting/eating normal.

I've never had something like this happen to my fish before.  Do I have a bacteria problem or a water quality problem?  And how would I go about treating this?

Thanks,
Phil

Answer
Phil,                                                        These are vey strong fish. Check your ph . They run 7.8 to 8.3.The 3 lakes they come from are directly fed by saltwater so they run high ph.I am sure you know that. If that is ok, I would add a broad spectrum antibiotic.....tetracycline... don't forget to remove your carbon for the 7 days and a 25% water change after its all over ..... he should be fine...it definately sounds bacterial....Good Luck, Tina