"I don't think they are dying - oh look, they are. Let me get some more bleach."
In defence of the 35-year-old, the court heard how she had been left to look after five children under the age of 10, a cat, a snake and the tropical fish following the breakdown of her relationship.
In a second video, Reynoldson said: "Oh no, they are all dying. Oh no, they are dead, look. Oh no, poor fish."
Oxford Magistrates' court heard how Fitzpatrick had moved out after Reynoldson accused him of having an affair.
She said she had been suffering from stress and anxiety following the end of their four-year relationship, which had been on-off since breaking down in 2013.
Fitzpatrick called police after the incident on January 9 and the RSPCA were called in.
She had sent two WhatsApp videos to her ex-partner threatening to harm the animals with rat poison. She claimed his pet snake "would be next".
Reynoldson, of Witney, Oxfordshire, told police in an interview that she did not know what the liquid was and she did not know what the consequences would be.
She admitted a charge of causing unnecessary suffering to protected animals, contrary to the Animal Welfare Act 2006.
RSPCA prosecutor Simon White said the crime was of the highest culpability and harm, and that bleach was not recognised as euthanasia for fish and they would have suffered before death.
Defending, Charlie Moss told the court Reynoldson was a victim of domestic abuse and was not drunk at the time.
Moss said: "This is an offence which was not directed at causing harm as such, but over anger at the breakdown of a relationship.
"She would like you to know that the breakdown really occurred because her ex was unfaithful but they had hoped they would be able to work things out.
"Ms Reynoldson felt that she was having to deal with everything he left behind including the animals.
"She has said to me that this was not a planned act, it was something which happened out of anger and frustration. She regrets her actions and has taken steps to address her emotional stability."
She had one previous conviction for common assault, dating back to 2007.
Chairman of the bench, Robin Syzmura, handed down a sentence of 18 weeks, suspended for 12 months.
An indefinite ban on keeping fish was imposed, and Reynoldson must pay costs of £300 as well as a £115 victim surcharge.