I don't know that I'd use baby oil. Not sure if it would be an issue as far as toxicity goes, but I bet it wouldn't be easy to clean up, especially since you don't want to use soap and water in an aquarium.
If you want to use something that isn't harsh, I'd use the white vinegar and razor blade.
If it's really tough, wait til spring and use diluted muriatic acid, just be sure to read up on it and use protective gear.