If you are using one of these drain mounted de-icers
http://www.tractorsupply.com/tsc/product/allied-precision-2002dp-universal-drain-plug-de-icer?cm_vc=-10005
They do not “screw” into an existing threaded drain plug. You just insert it in the opening. It has rubber gaskets on both sides with a threaded plastic “wire cover” and a big plastic “flange nut” screwed on the outside of the tank. Which squeezes the rubber gaskets tight to the tank walls to keep from leaking.
So all you have to do is drill the appropriate size hole to get the wire/plug through and the supplied fitting. Don’t make the hole any bigger than needed.
IME the “drain hole” fitting is poorly designed and made. They tent to strip out over time. Especially if over tightened. If you have a horse that “plays” in the tank it is EASILY broken. IME if a tank goes dry and horses are looking for a drink they will paw the tank by “instinct” and brake it. I’ve got several broken ones and or ones that the hole fitting stripped out and they leak.
I would be hesitant to use one of these is a low-walled tank
I FAR prefer the sink heaters. I’ve got ones that have lasted years.
If you do decide to drill a hole you can buy and retro fit a drain plug;
http://www.tractorsupply.com/tsc/product/rubbermaid-stock-tank-drain-plug?cm_vc=-10005
Make sure to check the hole size required for this before drilling. It is difficult to make the hole bigger after the fact. Even more so to make it smaller.