An alternative is Epsom salt, pure magnesium sulfate. Not aquarium salt. No dyes or scents added to it, be absolutely sure of that. You can buy it in the bath section at many stores and pharmacies for people.
Dose 1 tablespoon of Epsom salt per 10 gallons.
I would also soak the food in an Epsom salt mix. Do 1 tablespoon of Epsom salt mixed in 500mL of water.
Soak daily food in a teaspoon amount until No further excess water.
There are some studies that magnesium sulfate is effective against hexamita. If I couldn't get my hands on metronidazole, this is the route I'd take next.
Study:
Rainbow trout (average weight of 2 g) in fresh water experienced high mortality and were infected with a diplomonad intestinal parasite. Tanks of fish experienced an immediate reduction in mortality after an in-feed treatment with 3% Epsom salts for ...
pmc.ncbi.nlm.nih.gov
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