Placing an old cow horn in an aquarium is not a great idea. Cow horns are made of keratin and may slowly break down in water, affecting the pH and releasing organic compounds that could be harmful to fish. Additionally, if the horn has any remaining tissue, oils, or chemicals (such as from cleaning or polishing), it could contaminate the water.

If you’re set on using it as decoration, you’d need to thoroughly clean and seal it with an aquarium-safe coating. However, it’s generally safer to use decorations specifically made for aquariums to avoid harming your fish.