Okay so a little background on this, I drove a truck for a company out of Fargo ND for close to 10 years, Every truck in our fleet had 110 volt block heaters that could heat the coolant up to temps of 170 deg even in the dead winter it would keep the water temp above 100 and believe me it gets COLD up in that area.
Now that being said this is an idea, I have not tried it yet nor do I claim it would work in a full size production tank. Kats has a pretty good line of block heaters both freeze plug design and an external self circulating design with an internal thermostat that will shut the unit down if the water temp get above 170 deg (the reason I say you will need a thermo-controlled outlet)
Now these block heaters range anywhere from 400 watts all the way up into 2000 watts for the freeze plug type and 1000 to 2000 in the self circulating type.
More importantly however is that these are prewired and are properly grounded and sealed. If you combine one of these with a thermostat controlled power outlet it may be possible to have an heating system for a smaller DIY tank in the 150 gallon range for somewhere around $100 with a piece of mind of having a system that isn't likely to electrocute you or catch fire. Nor have to attempt to wire in a 220 supply to the tank.
Also when looking for a controller make sure to get one that is able to handle the amperage at about 200% load to insure that you do not overheat the system. To calculate amperage the best way is to use the internet for circuit breaker calculator
The reason I say to use a circuit breaker calculator is because a breaker uses heat to sense an overload or short.
This would mean that for a heating system using 1000 watts you need a controller able to handle at minimum 20 amps 10 amp work load plus another 10 amp safety cushion.
Please! do not ignore this cushion there is another factor that you need to understand. An 10 amp motor and an 10 amp heater are two completely different creatures when it comes to the draw on a system. An 10 amp motor needs 10 amps to start, but may run at 5 amps.
An 10 amp heating element however will continually use 10 amps the whole time it is on causing more heat buildup in the controller unit.
Again I have not tried this, its me bouncing my idea off a wall and seeing what others think of my logic with my own caution statement in case someone else tries it.