FWIW, I have some experience in dealing with chillers in large aquariums (>400 gallons). Over the years we have tried several configurations of redundancy in the chiller department for large tanks with intense lighting. The Following were our discoveries:
1. Dual in-line chillers w/ one strictly as a pass through backup.
The problem we ran into in this configuration was accumulation of gunk within the chiller. Over time, some amount of debris built up inside of the chiller andf then we we ran it once every other month or so to ensure its operation in the case of an emergency, the chiller was less effective because of the build up. We eventually abandoned this configuration.
2. Dual chillers running at 50% capacity
The problem here was the increased level of power consumption and maintenance. Two chillers are twice as much to maintain. One more than one occasion we had both chillers fail roughly at the same time, thus defeating the redundancy principle. These types of failures seem to happen at 3AM too, which meant my cell phone was going crazy with controller updates in the middle of the night.
The final solution we settled on was to have a second chiller essentially in standby. We place one chiller inline, with a second identical chiller nearby fed by a mag drive pump placed in the sump. This chiller and pump would only be turned on by the controller in an emergency situation. The controller would alert us of the temperature condition and the activation of the second chiller and we would then go over and swap out the inline chiller with the backup. We would service/ replace the failing chiller and then reinstall it in the backup configuration. Granted, adding a submersible pump to an aquarium adds heat to water you are trying to cool, but the solution was simple and it worked as a stop gap until we could address the situation.
