What medication are you using?
Is the fish still eating?
Is it still doing stringy white poop?
What is the water temperature?
Sick discus like it warm 30-32C.
Temperature has nothing to do with Hexamita. Hexamita is caused by dirty tanks and bad food. Discus need lots of plant matter in their diet and if they are fed a meat only diet they regularly get intestinal problems. Fish kept in dirty tanks will also have more health issues.
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Did you do a big water change, gravel clean the substrate and clean the filter before starting treatment?
If not, then that would be the start. Do the following.
Wipe the inside of the glass down with a clean fish sponge. This removes the biofilm on the glass and the biofilm will contain lots of harmful bacteria, fungus, protozoans and various other microscopic life forms.
Do a 75-90% water change and gravel clean the substrate. The water change and gravel cleaning will reduce the number of disease organisms in the water and provide a cleaner environment for the fish to recover in. It also removes a lot of the gunk and this means any medication can work on treating the fish instead of being wasted killing the pathogens in the gunk.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens so any medication (if needed) will work more effectively on the fish.
Increase surface turbulence/ aeration to maximise the dissolved oxygen in the water.
Make sure you remove carbon from the filter so it doesn't remove the medication.
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To work out the volume of water in the tank:
measure length x width x height in cm.
divide by 1000.
= volume in litres.
When you measure the height, measure from the top of the substrate to the top of the water level.
If you have big rocks or driftwood in the tank, remove these before measuring the height of the water level so you get a more accurate water volume.
You can use a permanent marker to draw a line on the tank at the water level and put down how many litres are in the tank at that level.
There is a calculator/ converter in the "FishForum.net Calculator" under "Useful Links" on the right of this page that will let you convert litres to gallons if you need it.