My betta is pale, and his head is turning kind of black. Help?

For a while now my betta has been acting kind of funny, he quit swimming and would just float around at the top of the tank. He's in a 5 gallon wish take, with a filter that doesn't really push out a lot of water, so there isn't much of a current in the water, he has a plant and we add the betta conditioner as it says to on the bottle. Well as the weeks have pes, he's been getting paler and paler. He now likes to sit on top of a floating flower that is submerged under the water about an inch or so. He used to spazz around the tank and bounce off the walls, so we added a parasite tablet, just half of one so it was suitable for the tank size. It seemed like it helped for a few days, well now he doesn't spazz out, but he quit swimming and his head is now getting really pale. The top of his head is turning black though, and you can kind of see through his nose. :( it makes me sad that he's sick and I don't know what to do. So if you can help me, anything would be appreciated. Thanks.