Quest as soon as possible the Torch of Alna (
https://wiki.project1999.com/Torch_Of_Alna_Quest), that will increase your fire pets HP as well as giving you instant cast reclaim energy.
In addition to what Nycon43 said, the fire pet innate DS is stronger than your own DS and is mana free, which is what makes it so valuable. When you will be chaining fire pets, make sure to hit pet back off till it buffs itself with DS, cause it doesn't pop with it on.