OK. This is NOT a pond. There's zero pond advice re: overwintering fish that applies to a 30 gallon container. ZERO. De-icer - no. Heater - no. Cover the pond - no. Combination of all three - NO. The water volume simply isn't great enough to prevent it from freezing solid. AND with our cold and DRY winters, it's highly likely you will end the winter with far less water than you started with - are you interested in refilling your container pond mid-January? Digging it out to check to see if it needs water? We honestly have that concern with a 4000 gallon pond - imagine 30 gallons.
I am also IL zone 5B - your container will 100% freeze solid and likely crack as a result. No way will ANY fish survive. I would recommend you deal with the fish however you choose - keep them indoors, re-home them, etc - and empty that container completely, or you won't have your mini-pond come spring.
Please understand I'm not dissing your little pond - I have a patio pond myself that I love love love. But having done this enough winters I can assure you that there is simply no way you can keep that "pond" running all winter.