Fish regulate their internal pH through acid-base homeostasis, which involves balancing hydrogen ion intake and removal. The body maintains pH levels through several systems - buffers in the blood, respiration through the gills, and excretion by the kidneys. There are four types of acid-base imbalances that can occur if homeostasis is disrupted: respiratory alkalosis, respiratory acidosis, metabolic alkalosis, and metabolic acidosis. The body aims to compensate for these imbalances through respiratory and renal mechanisms.