You need to answer the full array of Qs asked of all ill fish owners and then check out Disease Diagnostic Charts and Other Helpful Sites
Answer these Qs to ensure we have as much information as possible about your ailing fish and its environment:
1~What type of fish is afflicted? In addition, please describe what is wrong with the fish to the best of your ability (i.e. cotton like growth, bloated, etc.).
2~What are your tank parameters (ammonia, nitrites, nitrates, temp, pH)? Please give exact values.
3~ How large is the tank? How long has the tank been set up?
4~What type of filtration are you using? Please give the name and number (i.e. Fluval 304) and amount of gph
5~How many fish are in the tank? What kinds of fish are they and what are their current sizes?
6~When is the last time you did a water change and vacuum the gravel? How often do you do this? How much water do you remove at a time?
7~How long have you had the fish? If the fish is new, how did you acclimate it/them?
8~Have you added anything new to the tank--decor, new dechlorinator, new substrate, etc.?
9~What kind of food have you been feeding your fish, have you changed their diet recently?
The above Qs may help, but I really need to know the tank mates. Your info tells me, lake "malawi cichlid tank" but that's pretty broad. Venustus are open water fish and will not always know to hide when being attacked. I had one killed due to stress--she had been part of a Mbuna tank for 2 years and I added a fish from a different region (totally stupid, but didn't know about AA back then). When the new fish hit maturity, he went after the venustus.