Research the exact fish
A suitable temperature for one tropical species may be chronically stressful for another. Wild location, domestic strain and life stage may matter.
Use two points of confidence
Use a reliable heater with enough capacity for the room and an independent thermometer placed where you can read it.
Change gradually
When a species-specific breeding plan calls for a shift, make it slowly and monitor behaviour and oxygenation.
Plan for the room
Sunlight, furnace setbacks, summer heat and open lids can cause daily swings. Small tanks change faster than large ones.
Protect fry and surface-breathers
Covered warm air can matter for labyrinth-fish fry. Do not create a sealed tank; maintain safe ventilation and equipment.