It would seem like you are describing swimbladder. Swimbladder can be caused by a bacterial, viral or environment issues. The fish could have contracted the disease by being attacked by opportunistic viruses/bacteria if it has a weak immune system. The fish will only have a weak immune system if your water quality is bad. I suspect that the water changes you are conducting every 2 weeks are too infrequent because during that period the hardness (GH) and buffering capacity (KH; total alkalinity) will get reduced by the nitrogen cycle played out in your filter and by any plants that are photosynthesising. Thus, at the end of this 2 week period when you replace 25% of your water with fresh tap water, the GH/KH levels in your aquarium will be extremely different to the GH/KH of the tap water. A large difference may constitute a stressor on the fish.
For this reason, and providing the swimbladder has a bacterial/viral cause due to stress on the fish, I'd recommend conducting 25% - 30% W/C's weekly and look into getting some swimbladder medicine.
If, however, the fish that 'Jack' has consumed has disrupted the organ responsible for controlling buoyency, I'd recommend feeding cooked garden peas which have cooled down. They can help to release trapped gas.
In any case, I'd recommend you monitor your GH and KH levels of your tank water every 3-4 days during a given (and the GH and KH levels of your tap water) 2 week period and once a discrepency of 2-3 dH occurs between the GH and KH stats of the tap water and tank water, I'd go about doing a W/C. API test kits are the most accurate for doing such tests.
