Because the Moor's eye sticks out, it is prone to getting bumped and injured. What type of decorations are in the tank? I agree that you may be going through a mini-cycle from changing out the filter. It is usually a good idea to put the new filter in the tank for a week or so before you change it - this gives the good bacteria a chance to become established in the new filter and can help avoid a mini-cycle. You have three goldie's in the tank? Are there any other fish in the tank? What size is the tank? Just as a guideline, for three fancy goldies, you should have (at the very least) a 40 gallon tank. Larger is always better and a 55 gallon would be okay for the long term. Sometimes, a tank will be clear for several months - a couple of years and finally the fish in it will grow just large enough to overwhelm the current setup with
ammonia. That may be what happened. For now, I would suggest doing daily partial water changes with Prime (or another water conditioner) to help protect the fishies and get you through the possible mini-cycle. Good, clean, clear water should also help the Moor's eye heal. Generally, the goldies that have protruding eyes do best with only silk plants and smooth, rounded decorations. Their eye sight is not all that good, and it is very easy for them to bump into stuff and injure their eyes.
Keep us posted, and pics are always welcome!