A 6 wt rod can certainly handle a 3-5 lb bonefish. However, you may need to cast it 50-60 ft, so even if the wind is calm you want a fast-action rod that can double-haul well. Also, you want a reel with a tough drag and as much backing as possible -- preferably 200 yd. Replacing your 6 wt trout line with a tropical saltwater line is necessary at a minimum, but it may not be adequate if you put it on a typical freshwater 6 wt reel with a light-duty drag and 100 yd or less of backing.