Question

Warm water in aquarium thinking that the fish would become more comfortable unit however the fish died what can be the reason for this

Answer 1

Fishes are ectothermic vertebrate that means they cannot adjust their body temperature according to surrounding unlike human beings or other mammals and birds do.
Warm water in aquarium disturbs the fish internal body temperature and as they are unable to adjust hence they die.
Another explanation for the same is the concept of stenothermal and eurythermal organism. Eurythermal can tolerate wide range of temperatures and stenothermal can tolerate only narrow range of temperature. Fishes are stenothermal organisms and hence cannot tolerate wide range of temperature thus died