I've heard that bettas are sensitive to temperature changes - within "a few degrees" - but what does that mean? In a large tank, the water can vary by a couple degrees in different corners that's OK, yes?
For example - suppose your betta is in 79* water and a disaster happens and you need to net him into new water immediately. If it's 77* will that shock him? 75*? 73*?
Does it make a difference if the new water is warmer or colder? Starting from 79*, is 82* handled better/worse than 76*?
Or, if you're doing partial water changes, if he's in 79* water and you put in 81* water gradually over 20 minutes... no problem?
What about a more typical situation: he's in 79* water and you put in room temperature 71* water - so at the end of the 20 minutes his water has gone from 79* -> 75*?
What about day-night temperature swings, or small-heater temperature swings?