I agree that he was nipped, you can also see some damage at the end of the fin in the shape of another Guppy's bite. I've had Guppies nip fins before when moved from their main tank into smaller quarters. You should consider adding some Stress Coat+ to the water. That will help the fin regrow faster and more cleanly.
Also, don't change more than 50% of the water at once. If you change more than that, the parameters from the new water can be different enough from the water the fish were in before that it can cause the fish to go into shock, which is dangerous for them. The longer you've gone without a water change, the bigger difference the new water will be from the old water and the more likely the fish are to go into shock if you change a bunch of the water at once. The pH, hardness, and nitrogenous waste changes can all cause a fish to go into shock. So large water changes can actually be especially dangerous after a long time without one. If it's been months since a water change and you suddenly do a larger (over 50%) change, the fish can even be killed from the shock. Instead of doing large water changes after a long time without doing them, you should instead do 10-15% water changes each day for a few days. That will prevent the fish from going into shock. It will also mean that the fish can stay in the tank during the water changes, which will significantly lower their stress levels.
