Sorry about your loss.
Heater failures are fairly common.....never trust them.
Two common options
1) Use 3 or 4 small heaters that alone are not enough to overheat the tank. Odds of a failure are high, but odds that more than one will fail are fairly low.
2) Run a heater on a controller. Let the controller do the work, but set the thermostat on the heater just slightly higher.
Using a combination of #1 and #2 would be the best redundancy and protection against failure.