Ripped fins on several goldfish

Question
Hi Chris,
Recently, I noticed that most of my goldfish have ripped fins. This had never occur in my tank before. Can you tell me what is the problem and how can I cure it? I have a total of 9 gold fish in a 45 gallons tank. My filter is the Rena Xp3. Thank you so much for reading question.

Answer
Hi Quan;

How long has the tank been set up?
How often do you usually make water changes? How much water at a time?
How often is the filter cleaned? When was the last time the filter was cleaned?
How big are each of the fish?
Do any of the fishes' fins look good with no torn fins?
Do they seem to chase each other?
What is the water temperature?

Let me know as soon as you can...

At Your Service;
Chris Robbins