It can include someone who has lots of plant tanks, and he doesn't have the time to have CO2 in all of them because of the extra maintenance involved. At some point, money also becomes an issue. With many tanks, the electrical costs or equipment costs can become a significant factor, so cut backs in appliances is considered. This can involve heaters. In fact, I maintain several of may tanks without heaters.
Fish can tolerate fluctuations in temperature, pH, and water chemistry.
Certainly, Fish farmers don't heat their outdoor ponds, and except under extreme temperatures, the fish survive. The important thing is to allow for gradual change and to avoid extremes. Fish and especially plants are adaptable. It can include the someone that is always pushing the envelope, building upon the successes of others, or developing new approaches to meet a special need.