My experience with preset heaters.. max temp is between 76~78 degrees and every one I have owned has always stated that the most it keep up with is 7 degrees plus or minus of that range. If the ambient temp in your house is 68 degrees because you like it to be ice station zero then that unit will at best make 75.. 68 + 7 = 75. Taking into consideration it can be cooler in your house at night then the temp can sway up and down and that may stress poor fishy out. That leaves me with adjustable heaters. While not perfect by any means, they tend to control the temperature better for me. I have a digital and glass that I use to adjust and monitor my tanks with. I keep them around 80 for the most part.