Kirstie Trup and Katie Gee, both 18 and from London, had acid thrown on their faces, chests and hands.
The island's Police Commissioner Musa Ali Musa told the BBC that there was "no prime suspect" for the attack.
He said that a lot of people had been questioned and information gathered.
However no-one has been arrested or charged and investigations are continuing, Mr Musa said.
The police commissioner added that the reward, which is being offered by Zanzibar's government, was considered a large amount of money on the island.

He said it was the first time visitors to the island had been attacked in this way, describing the incident as "alien" to both the police investigating and the victims.
Officers had no idea as to the m
otive, he said.
'Unprovoked attack'
The two young Britons have been flown back to the UK.
Their plane was met by ambulances when it arrived at RAF Northolt in west London at lunchtime on Friday.
They had been taken to Tanzania's largest city Dar es Salaam for medical treatment following the attack, which took place in Stone Town, the old part of Zanzibar's capital, on Wednesday.
The women's families have said they are "extremely upset" by the "completely unprovoked attack" on their daughters.

The victims were two weeks into a three-week trip, volunteering for the charity Art in Tanzania.
Before they left Tanzania, the country's President Jakaya Kikwete visited the women at the Aga Khan Hospital in Dar es Salaam and promised that those responsible for the "shameful" attack would be found.

The BBC's Tulanana Bohela, in Dar es Salaam, said Islam is the main religion on Zanzibar, and in more remote parts of the island, away from tourist beaches, there are signs asking foreigners to respect the local culture and cover up.


However, most islanders depend on tourism for their livelihoods and are happy to see tourists, so there is little antagonism towards them, she said.

A statement taken from another article:

Ms Gee had been startled earlier in the trip when she was hit in the street by a Muslim woman for singing during Ramadan.

UPDATE:

Muslim group allegedly behind acid attack of British Jewish teens

Zanzibar police issue arrest warrant for extreme imam after two young volunteers burned with acid in street. Girls dressed modestly, hid Jewish background, say relatives. Religious figures point finger at fanatic Muslim sect 

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