Why is heater for aquarium mentioned as a solution that need to be purchased almost instantly by a person, who would like to have his own fishes?

Fishes are with no discussion an example of such pets that we like owing to miscellaneous reasons. Firstly, most of people associate them with tasty as well as healthy food. Consuming fishes is recommended by various experts and doctors, who in most cases say that their impact on the health of people is quite significant. However, this is not the only one association we have concerning the previously presented group of pets.