I agree it happens when you overfeeding the fish.Looks like a "poop trail" from constipation (usually caused by over-feeding in my experience).
Cleaning my goldfish tank is torturous, too much poop.No, in a goldfish that is normal poop. . They do it several times a day. Unlike other fish, goldfish have no stomach so it goes right through. I always love cleaning my goodie tank. Lol!
I have one but still hard to clean it.Use a gravel cleaner. You can buy them from pet shops or make your own from a 1, 1.5 or 2 litre plastic drink bottle and a garden hose.
Cut the bottom off the bottle and remove the lid and plastic ring from the top of the bottle. Throw these bits in the recycling bin.
Stick a garden hose in the top of the bottle and run the hose out the door onto the lawn.
Start draining water out of the tank and push the plastic bottle into the gravel and lift it up. Then move it to another area and push it into the gravel and lift it up. The gunk in the gravel gets sucked out with some of the water. When you have drained 75% of the tank water out, stop gravel cleaning and refill the tank with dechlorinated water.
Make sure any aquarium heaters or power filters are turned off before draining the water out.