Can an aquarium chiller be used instead of a heater?

I don't think that it can be a replacement for a heater since they function completely differently. For example, a chiller/cooler actually cools the water down in case it gets too hot for specific species (think mostly of saltwater tanks). A cooler, in theory, cannot heat the water at all since many don't have that function unless you buy a combo unit that does (which isn't normally used unless you need to process a lot of water through the same system).