I had two fish, but one got really aggressive, and bit the other one's tail off. The second fish died because it wasn't able to compete against the first, but if it had been living alone, could its tail have grown back?
Your guppy's tail most likely may have grown back depending on how damaged it was. Usually if there are nips, tears, or small pieces missing then with pristine water (correct ammonia, nitrite, nitrate, hardness, pH) and the fish is healthy and not too stressed, it will grow back. If it is completely bitten off, then this may not be a survivable injury.
It all really depends on two things:
- Extent of damage
- Living conditions (water, environment, stress levels, disease, diet).
I would have recommended a tank divider in this situation, as moving the guppy to another tank would have stressed it out. The diet would be better if it is a varied one and there are many medicines out there that promote healing.
CAUTION: Using medications on fish is best to do in a hospital tank (a separate tank, dedicated to healing) because they can destroy beneficial bacteria colonies. Some medications are also not effective so beware when buying (similar to cure-all remedies from the 1900s...)
Our company has a few fish and we actually saw a fish regrow its tail from a bloody stump. We didn't think that was possible. It is a stunted tail but irrefutably better than 3-4 months ago. Will provide pictures of the fish if possible.