It was the 1st dive of the day on a 35m wreck of the Folio off the coast of Ireland, we descended the shot line and at approximately 20m depth the air began to taste strange coming from my main cylinder. I initially put this down to either swallowing salt water or a previous bad taste in my mouth.
Upon reaching 33m at the rear of the wreck I paused to adjust my straps & weight belt and prepared to move off. My buddy had signalled OK and I signalled back to wait a second, I initially signalled OK & we then started to move away from the shot line and the taste was getting worse. I signalled to my buddy I was not OK and would be returning to the surface. Unable to convey to my buddy that my fill was suspect only that I was not happy with my regulator, he offered me my octopus which I declined, he then pointed at my pony cylinder and I swapped on to that regulator. The taste changed instantly (the pony was filled on a separate day by a separate dive company compressor). We returned to the shot line and prepared to ascend with me on my 3 litre pony keeping an eye on my pressure gauge & depth computer.
At 6m my computer indicated a 1 minute deep stop followed by a 3 minute safety stop. Both were performed fully while still on the pony cylinder.
We returned to the surface at which point I signalled to the dive boat I was not OK and I finned towards the line. Once on the boat I was de-kitted and took on some water. I did not require any oxygen and it took some time for the taste to go.
The next day other members of the group checked their cylinders and reported a similar taste / smell coming from their cylinders.
On return to the UK I had my cylinder inspected and serviced, they confirmed presence of sea water, rust and hydrocarbons confirming what I had tasted.
The training drilled into me by my BSAC instructors during my lessons and the confidence in my buddy pair both helped in changing a potential incident into no more than a diving anecdote.