The best and also harmless for any species cure for hexamita I've found is explained in the below link and I've used it successfully myself.
It involves soaking/damping fish food in 3% magnesium sulphate(Epsom salt) solution and feeding the fish with it twice a day for 5 days. It's pretty damn easy
3% solution can be achieved by dissolving a table spoon of magnesium sulphate in 500ml distilled water or even tap water. Then damp the food with the solution as much as it can take but without excess water and feed immediately. Please note that dumping the magnesium sulphate in the water column instead has no such effect.
The solution doesn't need to be refrigerated and one solution is more than enough for the entire treatment. So no need to make one each day. I make mine in a 500ml plastic bottle that has a dropper.
Do not increase temperature or fiddle with the tank in any other way. If it's hexamita, it can only be killed with temperatures above 34-35C, so unless your fish can handle that, there's no point.
I had a fish that was so far gone it already had a caved in belly and had lost balance and it not only got cured but fully recovered, although the recovery process took a few months. She was still eating though. Sometimes with hexamita they lose appetite and then the only option is to treat with meds such as metro or octozin in the water column. Octozin contains dimetridazole which is as effective as metronidazole but hexamita is best treated internally. I am not sure if the strength in octozin is enough. I have read myself some double the octozin dose for larger fish but the point is it's the same thing as metronidazole.
Here is the link explaining the treatment.
http://www.monsterfishkeepers.com/forums/showthread.php?339362-Treating-Hexamita-aka-Spironucleus
If this doesn't work, then your fish may have some other sort of internal parasites or worms and not spironucleus/hexamita.