Sushi when you change the filter, how much media do you change?
Because of the bacteria you should only change half the media at a time. Changing all of the media will cause a mini-cycle in the tank.
Also how much water do you change and how often? And how long has the tank been set up?
I re call you mentioning you were going to upgrade the tank for your goldfish?
What you could do, is either grab a test kit from a pet store, or take a sample of water to a pet store (they'll often test it for free), so that you can know what your water quality is .
If ammonia or nitrite are above 0ppm or nitrate is above 40ppm, then that could be a cause to the fish's behaviour.
If they're both 0ppm and nitrate is below 40ppm, then you can rule out water quality.
So if you find the water quality is the issue, then I would increase your water changes, to once a day, or once every two days, until the levels are okay. Testing water frequently during this period would help, so either use a test kit or pop to a store like I mentioned before.
Then once the levels are okay, 50% water changes per week, should help keep the water of a good standard.
If the water quality isn't the issue, then check the fish visually to see if you can spot anything unusual like it's described in the article I linked to.