Are you meaning the black coloration on his pectoral fins? Those could be his natural coloring.. as sometimes they change colors as they age, and sometimes they add "spots".
For a bowl that size, as mentioned above, a 100% water change every 4 days is okay to do, but recommended for that size unfiltered is 1 50% and 1 100% per week, using tap water and water conditioner. Water conditioners tend to work immediately, so there is no need to let it sit out. You won't want to use water that has gone through the filter as the filter removes beneficial nutrients they need- plain tap water and conditioner is all that is recommended.
Not enough water changes, and not enough water being changed brings on the risk of fin rot which will eat away the fins. It's hard to tell from that picture what exactly is going on if it's not the spots on his pectoral fins (the 2 small ones he uses to turn).
It looks to be possibly a 1 gallon, not filled all the way, and with a lot of substrate- if you don't mind me recommending that you make sure he has plenty of swimming space, plants and even a cave would be appreciated by them. They sell these guys in cups and they say they can live in them- but like every other fish they do have requirements and do prefer and are happier in places where they can hide in, sleep on leaves or in a cave.
Don't forget the heat, if you don't have a heater already as they are tropical fish and should be in temps of at least 76*F-82*F. Too cold of water will lower their immune system, slow their metabolism as well and in time they can become lethargic.
If he seems to just sit around most of the time, then I do highly recommend smaller/less substrate and some decorations, and heat- you will see a world of difference in their personality and energy.