Get a UPS... At least, if the power goes out, you will get about 1 hour to 4 hours if the pump is the only thing on it...
Have you tried a pingpong ball and a micro switch with a relay as a water level sensor? Use a (NC) Normally closed relay, the coil should be rated at 9Volts - 12Volts, and the contacts should be rated at 120 Volts @ 20 Amps. Single or double pole makes no difference, you only need one set of switched contacts.
Get the appropriate adapter for the relay power, I personally use a 12 Volt setup for all my non 120 Volt power stuff. Fans, El-Wire, relays, etc... The reason that you want to use low voltage in the tank for "Home-made" stuff, is because if you F*** it up, you won't be killed, or kill your creatures. So, the relay should be outside the tank, and all 120 Volt connections should be sealed properly. GFI and grounded plugs only... etc... yada, yada, yada... If you doubt your electrical skill, then never try this!
How it works. When the tank drain starts to get clogged, the water rises. When the water rises, this lifts the floating pin-pong ball up, equal to the level of the water. If the water level ever gets to a dangerous level, the switch gets depressed by the floating ping-pong ball, which sends power to the relay. The relay normally allows the pump to run, but will now the coil will have power, and the relay will cut power to the pump.
How to set this up.
(This assumes the sensor will be mounted so that it rests on the bottom of the tank, attached to a stone or mount.)
Get the water in your tank to the desired level. Lower the switch slowly into the water until the pump turns off. Mark the canister where the water level is at right now. Determine how much more the water can safely rise by flooding your tank a little. Record the flood distance from the bottom of the tank, to the top of the water. Mount the sensor so that the mark on the canister is the same distance from the bottom as the flood level. Test this setting and adjust as needed.
NOTE, The return hose from the pump should barely touch the water surface when the water is at its normal operating level. If the return hose from the pump extends deeper into the water, the water will siphon out when the pump turns off. If the hose is at the bottom, your tank will be empty. If your hose is at the top you will loose almost no water. When the water falls below the level of the return hose lip, air will fill the pipe and no more water will be siphoned. You can minimize this with the aid of a check valve, but that is only a temporary delay.