So today I've noticed my reedfish is kind of just drifting / floating around the top of the tank and it's definitely not normal for him. Any ideas if there's a specific reason for this?
Tough to say. Reedfish are physostomous meaning that their gut is connected to the swim bladder. They also have one lung that they can inflate and deflate by gulping air.
What is he eating? Perhaps it's due to an upset stomach an he is having trouble burping. Also, does your tank have good aeration? Perhaps he's trying to get as much oxygen as he can from the air.
Epsom salt won't hurt but it's a diuretic. It's typically used for distension from fluids. Might help the gut flora though! Probiotic food could also help such as those offered by Cobalt aquatics. A healthy stomach could mean a healthier swimbladder!