It does look like he has a touch of fin rot to me, so make sure you're keeping him in nice, clean water.
If you have a smaller quarantine tank to move him to, I would go ahead and do that. But if you don't it's not a huge deal since he's the only fish in the tank. I would start with aquarium salt treatment and see if he improves with that alone. To dose, measure out enough salt for 1 tsp. per gallon. Mix it really well in a cup of water to ensure it is dissolved. Do daily 100% water changes for a week and re-dose with salt each day. After 7 days the fin rot should be gone. It is not safe to use aquarium salt longer than 10 days maximum. If the fin rot hasn't disappeared, or significantly worsens during treatment, please do post in the diseases section and someone can advise you on more aggressive treatment. There are some strains of fin rot that are harder to treat than others. Your guy looks to have a pretty mild case so I doubt you'll have any problems.
I wouldn't worry too much about the sand; the aquarium salt will help take care of all the bad bacteria in the tank.