An aquarium fish excrete ammonia whereas a pigeon excretes uric add.
Ammonia is highly toxic and needs to be eliminated as rapidly as it is formed.
A fish living in water have immediate access to water and can eliminate ammonia easily and directly.
Ammonia being soluble in water requires a large volume of water for its elimination which a fish can afford.
A bird has very less access to water and can conserve water in its body by eliminating insoluble uric acid crystals.