The pheromones are a factor, but nitrate build up is a much larger factor. This is where keeping fish in tanks, and aquaponics differ. A balanced system should eliminate the need for the frequent water changes needed in aquarium raised fish, or aquacultured fish.

The fish in you outdoor pond are likely growing better because of less nitrate concentrations. Your pond is not unlike an aquaponics system. You probably have some plants. Either intentionally planted, or some form of algae. Both will remove nitrates, and allow for better fish growth. Also the larger volume of the pond will help lower concentrations.

If you get a test kit, and test your indoor comet tank for nitrate, you might be surprised at the nitrate levels. Under 40 is considered optimum for most tank fish.

With my indoor tilapia in non aquaponic systems, I recently got lazy, and skimped a bit on water changes. I actually noticed that the fish became lethargic, and less aggressive at feeding time. Nitrite, and ammonia were fine, but nitrate was over 80. After a large water change, the fish were much more vigorous.