Help why does my goldfish constantly sink to the bottom of my tank but come up to eat. Is he just overweight or is it worse
One of the causes to this is bad water. The bad waters can cause his swim bladder to fill with blood gasses and therefore causing it to sit on the bottom. The best thing to do right now is to change the waters and consult a vet for more details.
my fish usually did that when they would die soon..
Bad water if he is doing that you should clean the tank every 2-7 days this is a sign of him going to die soon but if you clean the water often he might live a week longer
He could also just be a natural bottom feeder