Goldfish pressed to bottom indicates nitrate levels are too high, or a large water change was recently performed. Nitrates rob the blood of oxygen, causing pressure on the body. The pressure of water reduces the pressure caused by the toxin.
Perform small water changes frequently, instead of large water changes infrequently. Nitrate levels should be consistent, with no dramatic changes. Safe levels are 20 to 40 ppm, however, lower is better
Encourage the growth of algae, as algae feeds on nitrates, keeping levels lower. Removing large amounts of algae from tanks or ponds typically causes a big spike. Algae is also a very nutritious food for the fish