About to fill the tank with fire...

[Cristy Keister]

2-3 degree swing over the course of a day wouldn't hurt. 200w heater in a 20g is more than enough to keep the tank stable... except at water change time. If you do a wc with cold water and then the big heater rapidly brings the temp back up, that would be unnatural and possibly stressful. I may be reaching, but big temp swings are known to bring on bouts of ick.

[JMLenke]

I try to do a feel test to get it close to similar, again no issues in other tanks over the course of 10+ years.

This tank is honestly killing me with issues.

[RTRJR]

Environmental issues can affect tanks. My set of 5 smallish tanks at the west end of the fish room are cooler in winter than they should be, so they have heaters and those work harder as they back to the concrete foundation wall which is less insulated there. They are backed with stryro so the do not radiate to wall so much. It helps a tad. If there are drafts, jacketing the non-viewing faces with some insulation will help, but at some human risk in case of fire.