The recommended dosage seems to be:
1/8 flat teaspoon (around 0.5 ml) per 5 US gallon within aquarium (long term)
1 flat tablespoon (around 15 ml) per gallon for 10 minute bath (short term)
Epsom salts can be used to cure dropsy, bloating and internal parasites in fish.
Source:
Singapore Maths Tuition 