Alba Duran poured it from a water jug into a beaker so Joan Blaber, 85, could take her nighttime medication.
A coroner is trying to establish how the floor cleaner got into the jug on her bedside table at the Royal Sussex County Hospital in Brighton.
The retired shopkeeper was left "fighting for breath" soon after taking a sip and died six days later, an inquest heard.
Ms Duran, who was on duty that night, sobbed as she told how she was the one who gave her the drink, not knowing what was in it.
She said she delivered tablets to patients in the ward and left Mrs Blaber until last because she was asleep.
Although the lighting on the ward was dim, there was enough light to see what she was doing, she said.
The staff nurse told the coroner: "I said, ‘I have to wake you up to take your medication. I’m sorry I know you’re in pain.'
"There was a little bottle of summer fruits squash and a little beaker on the table so I put in some squash.
"I only put in a little bit of the water in the green jug."
Ms Duran said the OAP took her pills and lay back down.
She said: "Nothing seemed to be wrong, nothing seemed to be out of normal. I think she was quite tired.
"I didn’t smell anything."
Around 15 minutes later Mrs Blaber was seen to be coughing, the inquest heard.
Another nurse went to give her some water from the jug but when they poured it into the beaker they saw it was yellow.
Nurse Duran said: "I went to Joan. I wanted to see what had happened. I took the jug. I had to see myself that it wasn’t water.
"I went to the cleaning cupboard and I wanted to see what was inside the jug."
She said she poured it into a clear pot and saw that the liquid was yellow and smelled of lemons, then found a five-litre bottle of Flash on a trolley.
The hospital called the National Poisons Unit to see what medical treatment should be administered to someone who had drunk it.
But she died of respiratory failure caused by pneumonia brought on by drinking the Flash.
At the end of giving her evidence a weeping Nurse Duran took the hand of Mrs Blaber’s sister Rosemary and her son Gary before being helped from the court by colleagues.
Earlier the inquest heard a cleaning store cupboard was normally locked but was left on the latch on the day of Mrs Blaber was poisoned.
And a cleaner said a second elderly patient might also have drunk Flash at the hospital.
Daniel Gonzalez said noxious products were often left unattended and claimed to have heard a dementia patient once drank some.
Coroner Veronica Hamilton-Deeley has warned it could remain a mystery.
She said: "Even the most rigorous police investigation hasn't ascertained what happened. How can a patient ingest cleaning fluid?"
The inquest continues in Brighton.
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