Poo normally doen't stick to a fin. Usually it has got the same colour like the food they ate and the shape like tiny sausages 

I think the black spot you are worrying about can be ammonia burn and the missing scales due to poor water quality.
I mentioned about the Nitrogen Cycle in an other post replying to your question.

Take a sample of your tank water to the pet shop and get it tested, but write the results down, I mean the exact numbers and let us know. We can tell you then what may be wrong.

Don't be satisfied if the staff there tells you "..all is fine.." Get them to tell you the exact results and they should show you the water samples and the colour charts so you can compare too and learn how to read the test results as you may be able to perform those tests by yourself eventually