Hi everyone! I've been on this site so much since setting up my tank a month ago, and I have learned so much! Any question I might have had, its already been answered. Until now, that is. (Or, it has been answered but I couldn't find it.) This is a heater question. I have a 20 gallon tank with several live plants. I currently have one bettta, 12 glow light tetras, and 6 cories. Everyone seems to be very happy. I just added a second filter because AqAdvisor said I was under capacity. Now I am running a TopFin 10 and a MarineLand Penguin 125 (it was second hand). According to AqAdvisor I still have room for in my tank for the right fist. I was thinking about getting a swordtail. However, I am not currently heating my tank. The light above my tank keeps the water at around 23 or 24 degrees celcius. When I come in in the morning (my tank is at work) the temp has usually dropped to about 20 or just below. The petstore says I need to heat my tank in order to keep a swordtail in there, but I am worried that putting a heater in my tank will overheat things. I have a Tetra 100w Preset Heater I also got second hand from the same place as my second filter (along with a 36 gallon I am excited to set up at a later date!) I googled it an apparently the preset temp is 78 degrees Fahrenheit (25.6 degrees Celcius). AqAdvisor suggests keeping my tank at 24 degrees. Should I add the heater? Will it do any harm? Is there any chance of my tank overheating? If I don't add it, can I still get tropical fish?