A commonly used starting point is roughly 3-5 watts of heating per gallon, adjusted up if your room runs cool or the tank sits in a drafty spot, and adjusted down for a warm, stable room. A 20-gallon tank in a normal room temperature home typically needs 75-100 watts to comfortably hold a tropical range of 76-80°F.
Rather than one large heater working near its limit constantly, splitting the total wattage across two smaller heaters on opposite ends of a larger tank distributes heat more evenly and, critically, means a single heater failure won't cause as dramatic a temperature swing before you notice.
A heater stuck on is far more dangerous than a heater that's slightly undersized. Buy from a brand with a reliable track record, not just a low price.
Always verify actual water temperature with a separate glass or digital thermometer rather than trusting a heater's built-in dial, which can drift several degrees off over months of use. This is the single most common way heater problems go unnoticed until fish are already stressed.
A heater positioned near the filter outflow, where water circulation is strongest, distributes heat far more evenly through the tank than one tucked in a low-flow corner. Vertical placement, fully submerged per the manufacturer's guidelines, also matters — many heaters aren't rated to run safely if partially exposed to air.
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