IAL gives off beneficial properties when set in the water. The properties boost the fish's immune response and makes his tissue a little more sturdy. They will also taint the water light to dark brown based on how many leaves you put in. It's usually 1 leaf per gallon, I recall.
The grades the leaves are grouped in are just their appearance, A leaves are full; with no holes. C leaves tend to have some rips and holes. But nonetheless, they do the same job.
Since he's biting, I only suggest you do clean water. Because if you try to treat him, he'll just keep on biting..