Fluid formation after bypass surgery may occur some times and usually it settles in about 4-6 weeks time, once the raw areas inside the chest heal. As you already mentioned that the fluid has become much less after tapping and the chances are, as time goes by, fluid accumulation is unlikely to happen. As per your part, you can only do deep breathing and physiotherapy exercises of the chest and not worry about this accumulation, as it might not have any long term consequences.