Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Call emergency services now if the person has trouble breathing, severe wheezing, blue or grey lips, collapse, a seizure, severe confusion, or cannot be awakened.
Emergency help is also needed for rapidly increasing mouth or throat swelling.
And for anyone, someone who:
Contact a poison centre promptly after any intentional swallow, an accidental swallow of more than a trivial taste, exposure to an unknown or concentrated product, or symptoms after breathing fumes.
Do not wait for symptoms before calling if a child may have swallowed fixer.
Seek urgent assessment if pain, redness, blurred vision, or sensitivity to light continues after thorough rinsing.
Get urgent medical care for persistent vomiting, severe chest or abdominal pain, drooling, trouble swallowing, bloody vomit, worsening cough, or shortness of breath.
Skin that blisters, turns pale or dark, or remains painful after rinsing also needs assessment.
Routine follow-up may be appropriate after initial treatment if mild coughing, skin irritation, stomach discomfort, or swallowing difficulty does not steadily improve.
Follow the timing recommended by the poison centre or healthcare team.
The article also says
An eye splash needs immediate rinsing.
Every sign above is quoted from the sofidex article on Photographic fixative poisoning. If you are not sure, err upward — it is always reasonable to be checked.