How do freshwater fish maintain homeostasis when the pH of the water is changed

homeostasis means same state.

homeostasis actually means the ability to maintain stability

Fish, basically can only stand a slight change in pH change in their ecosystem otherwise they die.
An example would be goldfish. People have to clean the bowls and replace the water otherwise fish poo contaminates the water and changes the pH causing the fish to die.

Brachiosaurus was (on average) 75ft long. it's shoulders were 21ft off the ground, the upper arm-bone was 7ft off the ground. it weighed close to 90 (.U.S.)tons. this is the equivalent to 12 grown elephants.