What disease in fish can be caused by excess absorption of dissolved gases in the water?

Gas bubble disease is the correct answer as it specifically refers to a condition in fish that arises from excess absorption of dissolved gases, primarily nitrogen, in their bloodstream. This condition occurs when fish are exposed to supersaturated water, leading to the formation of gas bubbles within their bloodstream and tissues. The bubbles can cause distress and tissue damage, as they interfere with normal physiological functions, potentially leading to death if not resolved.

The issue is particularly prevalent in environments where water is artificially aerated or where there are frequent fluctuations in temperature and pressure, resulting in increased levels of dissolved gases. Understanding gas bubble disease is crucial for maintaining fish health, especially in aquaculture settings where water quality is closely monitored.

The other options, while representing various fish diseases, pertain to different causes and mechanisms. Fin rot is a bacterial infection affecting the fins and tail, Ichthyophthirius multifiliis refers to a protozoan parasite commonly known as "ich," and dropsy is a condition that results from fluid retention due to various underlying diseases. None of these conditions stem from the excess absorption of dissolved gases, making gas bubble disease the unique correct choice for this question.