Congestive heart failure (CHF) is a very common heart condition in older, small breed dogs. Dogs with CHF almost always die of their disease or complications associated with medications used to treat CHF and therefore this is a significant health concern for dogs. Although medications can increase the quality and quantity of life, additional treatment approaches are needed to improve the outcome for affected dogs. Blood chloride levels are often low (hypochloremia) in CHF because of diuretic treatment and this is a marker for advanced CHF in people and in dogs. Hypochloremia also appears to contribute to disease progression and therefore restoration of blood chloride levels is considered a therapeutic target in people. This study seeks to determine if oral potassium chloride supplementation can restore blood chloride levels in dogs whose values are low. If this study shows that acetazolamide increases blood chloride levels in dogs with CHF that are treated with standard of care medications, then investigators will plan a long-term study to assess the effect of this intervention on survival outcomes. This study has significant potential to improve the lives of dogs living with CHF.