Can you get hold of Epsom Salt? If so you can, dose this.Start with a dose of 1 tablespoon in 10 gallons and then watch the fish. If this doesn't help the first day, then increase the dose to a 2nd tablespoon. The thing to be careful about is that Epsom salt raises the hardness (TDS) of the water and doing this too rapidly can be harmful to fish as it takes them time to adjust to the change (osmoregulation is the issue). I would say the max level one should reach for Epsom is 1 teaspoon/gallon.
The other thing to consider is why is the fish constipated. I could merely be its plugged from over eating or not enough veggie matter in its diet. But consstipation or swim bladder issues can also be caused by an internal infection or internal parasites or worms. If the fish is a recent acquisition, especially if also a recent import, I tend to look to internal parasites/worms and medicate accordingly. If its a fish I have had a while I tend to assume its likely a bacterial infection and look to antibiotics. Unless you feel strongly now that meds are needed, I would try the epsom for a few days and if it doesn't help, consider medication as well as the Epsom.
