"fast deteriorating" is a correct and usable phrase in written English.
You can use it when you are describing a process or condition that is rapidly becoming worse. For example: "The water quality in the river is fast deteriorating due to pollution."
But the situation was fast deteriorating.
Even the best preserved pages of "Little Nemo in Slumberland" were fast deteriorating.
"Our request is premised on the alarming and fast deteriorating security situation in Egypt," Ghana officials wrote.
If they cannot nudge the government into even token concessions, another downward lurch in the spiral of violence is likely.The economy is fast deteriorating.
Take a case in point - cucumber growers, who need to preserve their fast deteriorating food as soon as it is picked.
Flimsy silt and clay flood defences, at least 50 years old and fast deteriorating, proved no match for the forces of nature.
"Poor conditions and standards that go unchecked can fast deteriorate into forced labour.
The Palestinians call their murders an intifada, though unlike the first, unarmed, uprising of the 1980s, this one fast deteriorated into a campaign of atrocities against civilians.
Despite knowing that his condition was terminal and his health fast-deteriorating, he not only bore it with good humour, but bravely continued to work to the limits of his capacity over the following months: his article for the January-February 2009 issue of New Left Review provides a succinct account of how he interpreted the origins of the financial crisis.
Then, on 28 June the family wrote on Facebook: "Bradley is deteriorating fast, his temperature is going very high his breathing very fast his oxygen levels low.
He was deteriorating fast.
