Drain on water supply could be housed in vacant properties, said Scottish Water
Scottish Water are continuing to search for the reason behind a "significantly higher than normal" use in the supply.
The utility company asked owners or those responsible for vacant properties to regularly check on them so incidents can quickly be repaired.
Engineers have been trying to isolate the issue and have been working in Fetlar to locate and repair leaks.
They also said the water network is being isolated overnight to allow water storage on the island to recover and maintain daytime supply to customers.
In a statement Scottish Water said: “The great majority of issues we are finding are on the supply pipes and plumbing that serves homes, businesses, community buildings and field troughs. Locating and addressing leaks quickly will minimise damage and help us in maintaining water supplies for everyone."