(1) If you use the dissected 6-gill cover, you will find bright red gills. Gill is an important respiratory organ of fish, and the gas exchange between fish and the external environment is mainly completed by gill. Fish keep swallowing and spitting water, mainly for breathing. Compared with inhaling water, the exhaled gas is more carbon dioxide. If we dissect the cut gill cover, we will find bright red gills. Gills are made up of many thin gills.

(2) The respiratory organ of fish is gill. Water flows from the fish mouth into the gills, and then flows out from the rear edge of the gill cover. When water flows through the gill filament, dissolved oxygen in the water enters the blood vessels of the gill filament, while carbon dioxide is discharged into the water from the gill filament, and the two exchange gas. Therefore, the water flowing from fish gills has lower oxygen content and higher carbon dioxide content than the water flowing into fish from mouth.

Verification method: put the fish in BTB solution, BTB is bromothymol blue indicator, which is harmless to the fish in a short time. Because BTB solution changes from blue to yellow when it reacts with carbon dioxide, we should pay attention to the color change of BTB solution. If the color turns yellow, it means that the fish exhaled carbon dioxide.

(3) In order to verify whether BTB solution will affect the life of fish, we can observe and record the opening and closing times of fish mouth. There is something wrong with this experiment: there is no controlled experiment.

So, the answer is: (1) capillaries;

(2) yellow; Carbon dioxide;

(3) The opening and closing times of the fish mouth; There is no controlled experiment;