The second law states that energy can't be created or lost, only converted into a different form. If the capsule was a sealed environment and nothing - not even sunlight or electricity derived from sunlight intruded - there would be no change in the mass of the capsule.
What would have changed was that the energy present had been degraded ie entropy had increased. This would be in the form of heat produced as the animals respired. The second form would be found in the conversion of the calories in the feed into a low-value waste-product. Without the addition of more electrical energy or calorific inputs the system would degrade to the point that it would not longer support life.
Its mass would not change.