Can aquatic plants reduce the acidity caused by fish breathing in a tank? Fish release carbon dioxide as they breathe. That carbon dioxide dissolves in water and makes it more acidic over time. You set up three tanks with five guppy fish each. One tank has no plants. The second has two seaweed plants. The third has four. You measure the pH every hour for eight hours. The tank with no plants grows steadily more acidic. More plants keep the pH stable or even push it higher.