Fish don’t sweat to regulate their body temperature but they do have strategies for thermoregulation (www.fishlady.us).
Thermoregulation is the process by which an organism controls its internal temperature. Fish have many different mechanisms for regulating their temperature. Most fish are ectothermic, using their environmental temperature to manage their body temperature, but some fish are endothermic, having the metabolic ability to internally manage temperature. Poikilothermic fish are ectotherms which have no control over their body temperature; their core body temperature conforms to ambient temperature. Eurythermic fish have evolved to survive in a wide range of environmental temperatures and stenothermic fish have evolved to survive in a narrow range of environmental temperatures.
Thermoregulation is very important for fish because temperature influences the function of many organs and the rate of many metabolic processes. Most fish species have evolved to survive within a specific temperature ranges; outside that range, enzymes can degrade, organs can fail, and the organism can die. Understanding thermoregulation for fish species is particularly important when considering implications for climate change.