The aquarium heater is the single most likely piece of equipment to kill a tank. A stuck-on heater cooks fish and coral within 6-12 hours. A failed heater drops temperature 8-15°F overnight, triggering ich outbreaks. Every reefer eventually loses livestock to a heater - the goal is to delay that day with proper sizing, redundancy, and an external controller.
50 gallon tank = 250 watts. 100 gallon = 500 watts. Splitting the wattage across two heaters (2x 250W instead of 1x 500W) provides failure redundancy: if one sticks on, the other can't cook the tank alone. If one dies, the other prevents catastrophic cooling.
Run heaters through an Inkbird ITC-308 ($35) or a tank controller (Apex, Hydros). The external controller bypasses the heater's built-in thermostat - if the heater fails on, the controller cuts power at 80°F. Single most important $35 you spend on a tank.
Eheim Jager (workhorse, $30-60, glass body, accurate +/- 0.5°F), Cobalt Neo-Therm (impact-resistant plastic, flat design, $50-80), BRS/Bulk Reef Supply Titanium (titanium element controlled by Apex, $80-150), Finnex Titanium (budget titanium, $50-100). Avoid: cheap Amazon glass heaters that crack at the seal.
Glass: cheaper, more accurate built-in thermostats, but cracks if it touches gravel or a fish lays against it (catastrophic - it leaks 120V into the tank). Titanium: nearly indestructible, slightly less accurate built-in thermostats but you should be running an external controller anyway.
Bench-test new heaters in a 5-gallon bucket of water with a known-accurate thermometer. Verify the heater holds 78°F within 1°F before installing in the tank. Out-of-box heaters are often 2-3°F off the dial setting.
A separate digital thermometer (TempStick, Inkbird IBS-TH1) sends a phone alert if temperature exceeds 82°F or drops below 75°F. Catches stuck-on heaters before they cook livestock.