If it's just a line, it means its broken (fuzzy/patchy means fungus). If you have something like a betta cup, you could attach it to the tank (with electrical or packing tape) and keep him warm that way (you can even put small holes in the bottom or sides so that it remains free of ammonia (the entire point of a breeder box). if you can't find anything to do that, keep his bowl in a warm area and try to find a good heater. Nat Geo for PetSmart makes good small tank heaters. Just make sure you get a thermometer as well to judge the temp, especially if you get one without an internal thermostat.