Choosing a heater: wattage, placement, and when two are better than one
The standard rule is 1 watt per liter for tropical tanks in a heated home (18-22°C ambient). A 75L tank gets a 75W heater. A 200L tank gets a 200W heater. The heater calculator refines this based on your actual room temperature and target tank temperature, which matters if your fishroom runs cooler or warmer than the house average.
That rule works for most setups. The details below cover the cases where it doesn't.
Sizing
The heater's job is to overcome heat loss from the tank to the room. Heat loss depends on the temperature difference between tank and room, the tank's surface area, and whether the tank has a lid (a lid reduces evaporative cooling by 30-40%).
For a tank in a room held at 20°C targeting 26°C (a 6°C differential), 1W/L is adequate. If the room drops to 15°C in winter (an unheated garage or basement fishroom), the differential doubles and you need 1.5-2W/L. If the room is consistently 22-24°C (a heated apartment in a warm climate), 0.5-0.75W/L works.
Undersized heaters run continuously and never reach the target temperature. They won't burn out faster than properly sized ones (the thermostat never cycles off, but the heating element isn't overdriven), but the tank stays cooler than intended. Oversized heaters reach temperature quickly, cycle on and off frequently, and create small temperature swings around the setpoint. Both are acceptable; modest oversizing (25% above the calculated wattage) is safer than undersizing.
Two heaters instead of one
The most common heater-related fish death isn't from a heater failing off (the tank cools gradually, fish tolerate 2-3°C drops overnight without acute harm). It's from a heater sticking on. A 200W heater with a failed thermostat runs continuously and can raise a 200L tank from 26°C to 34°C+ within 12 hours. That kills everything.
Two 100W heaters instead of one 200W heater mitigates this. If one sticks on, it doesn't have enough wattage to overheat the tank alone. The tank might rise to 29-30°C before stabilizing, which is stressful but survivable for most tropical species. The other heater, still cycling normally, holds its end of the thermal load.
The tradeoff is cost (two heaters and two thermostats instead of one) and clutter (two units in the tank). For tanks above 150L, the safety margin is worth it. For smaller tanks, a single quality heater with a reliable thermostat is sufficient.
Placement
Heaters need water flow across the heating element to distribute heat evenly and to give the thermostat sensor an accurate reading. Place the heater near the filter outlet or return, where flow is strongest. If the heater sits in a dead zone, the water immediately around it heats up, the thermostat reads that warm pocket and shuts off, while the rest of the tank stays cold.
Submersible heaters can be mounted vertically or horizontally. Horizontal placement along the back bottom glass puts the heater in the warmest-rising convection current and gives slightly more even heating in tanks without strong circulation. Vertical is fine if there's adequate flow.
In-line heaters (installed on the canister filter return tubing) eliminate the heater from the tank entirely. The water heats inside the unit as it returns from the filter. Clean look, no equipment visible in the tank, and the flow through the unit is guaranteed by the filter pump. Downsides: only works with canister filters, and if the filter stops, the heater has no flow across it and can overheat locally (most in-line heaters have a flow sensor cutoff to prevent this, but verify before buying).
Temperature controllers
An external temperature controller (Inkbird ITC-308 is the standard budget option) plugs between the wall outlet and the heater. It has its own temperature probe in the tank and cuts power to the heater if the water exceeds a set maximum. This provides a hard safety ceiling independent of the heater's built-in thermostat.
For tanks with expensive livestock (discus, high-grade shrimp, rare cichlids), a temperature controller is cheap insurance. For a standard community tank with a quality heater, it's optional.
Brands and longevity
Heater brand recommendations change as manufacturing quality shifts. The general advice: buy a heater with a visible thermostat adjustment (not a preset), a suction-cup mount that actually holds, and a warranty. Titanium-element heaters last longer than glass-tube heaters in hard water because calcium deposits don't crack them. Glass heaters are cheaper and work fine in soft to moderate water.
Budget heaters work but fail sooner. Mid-range heaters last 3-5 years in typical use. Premium heaters last longer and have better thermostat accuracy. Over the heater's lifespan, the cost difference between budget and mid-range is small enough that mid-range is usually the right call.