Heart Attack: This may cause death, although if it doesn't symptoms will include - Lethargic behaviour, Not eating, Paler than usual skin, strange swimming stroke. Heart Attacks can be caused if the water is too warm, their heart rate is affected by their external surroundings. If it is cold then their heart rate will be slow, if it is hot their heart rate will be fast, if it's too hot the heart may not be able to take the strain
Stroke: This is when blood flow to the brain is decreased due to a blocked vessel or burst small vessel. Symptoms of this may include Lethargic behaviour, not eating, unable to move back or front legs/arms, continuous trembling of a limb or unable to regulate balance in the water. Strokes can caused by stress caused by handling or movement of the tank, they can also be caused by feeding too much fatty foods like bacon fat etc., and temperature fluctuations can also be a cause.
Very sad, I'm sorry.