Is it permissible to have a snake as a pet, especially if it is harmless, and will be properly cared for in a large tank, etc?
In the name of Allah, Most Compassionate, Most Merciful,
To keep impure animals such as a pig, and harmful animals such as predators, lizards, snakes, etc as a pet is not permissible.
In a Hadith recorded by Imam al-Bukhari, the Companion Abd Allah ibn Abbas (Allah be pleased with him) says:
“…Then suddenly a snake sprang at us. The Messenger of Allah (Allah bless him & give him peace) said: “Kill the snake.” We hurried to kill it, but it escaped us. The Prophet (Allah bless him and give him peace) said: “It escaped your evil and harm, and you escaped its evil.” (Sahih al-Bukhari, Chapter on what kinds of animals can be killed by a Muhrim in the Haram)
The Messenger of Allah (Allah bless him & give him peace) also ordered the killing of a Crow, Kite, Scorpion, Mouse, Dog, Lizard and other harmful animals. The jurists (fuqaha) mention that, it is permissible to kill animals
which are harmful to ones self or ones belongings (such as pests). (See al-Fatawa al-Hindiyya, 5,397)
In light of the above, it can be deduced that it is impermissible to keep a snake as a pet, rather it should be exterminated.
If the snake is, however, harmless and the poison and venom has been removed and extracted, as you have also mentioned, then it should be permissible (and Allah knows best) to keep it, although detested, thus it would be better to avoid keeping it.
And Allah Knows Best
[Mufti] Muhammad ibn Adam
Leicester , UK