What is in the tank? Is it glass or plastic or acrylic? You can use a strong salt solution, vinegar or boiling and or baking. Don't use soap, it is harmful to fish, even traces of it can kill future fish. I am not sure about alcohol, you can use bleach too. Make sure you rinse with dechlorinated water, add extra dechlor as bleach is made of chlorine. Rinse, rinse, rinse. and if you think you might have missed a trace of what ever you used, rinse again. Letting the tank sit dry for a while might work. Especially if you let it sit in the sun.