The symptoms you describe are definitely an indication that the fish have Ick.
The treatment you are giving is fine but remember to change at least 15% of the water daily vacuuming the gravel at the same time. The purpose of vacuuming the gravel is to help rid the tank of the Ick parasites that have fallen off the fish. Aside from the medicine, aquarium salt and partial water changes there's not much else that can be done to treat Ick. When using aquarium salt you should also be careful when there are tetras in the tank as they sometimes do not take kindly to it.
As for brackish water, yes you are correct when say that goby dragons are brackish water fish. To make water brackish you would need to add 2 tablespoons of marine salt not aquarium salt per every gallon of water. So a 110 tablespoons of marine salt would be needed to make a 55 gallon tank brackish.
Making the water brackish would be harmful to the other fish as all the other fish you mentioned you have are not brackish water fish and would therefore most likely not survive as they are freshwater fish.
The freshwater fish should be housed in water that has a ph range of 6.8-7.2.
Also the goldfish should be housed seperately as they do not require the higher water temperatures as the other freshwater fish you have. Goldfish thrive in a water temperature range of 62f-72f degrees whereas the other freshwater fish would need a
water temperature range of 74f-78f degrees.
Best wishes and please let me know if you have any questions.
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YOUR TROPICAL FISH RESOURCE
OVER 20 YEARS EXPERIENCE W/TROPICAL FISH---BREEDING, MAINTAINING, CURING
If the dragon is not kept in a brackish water tank it will not be ok. It's chances of survival will be greatly lessened if not housed in brackish water.
The acid you speak of I assumed to mean acidity in the water which basically has to do with ph. For a brackish tank a ph of around 8.0 is good.