Note: This is my experience. I am not responsible for your losses (of any kind) by you doing the same as I did.
First, let me explain why I tried Marine Salt:
In addition to my freshwater tanks I also keep a Brackish tank (this is a low salinity tank which contains fish that live where rivers meet the ocean, so it is diluted sea water), so I have sea salt readily available. My tap water used on water changes is quite hard and full of nutrients, the pH is also very high: This meant that using marine salt could not significantly raise my pH in a way my freshwater fish were not already accustomed to. Also, the weather was bad, I had salt in the house and could begin treatment immediately.
How did I get Ich?
My main freshwater tank has a 6-inch Oscar who is growing fast. I bought feeder fish and rather than eating them immediately like he always had, he left some alone for over a week. I noticed the Ich first on the feeder fish but obviously the way the parasite works it meant that it was already too late for the rest of the tank. I had also made the mistake of moving plants around between my tanks during the time those feeder fish were alive and this meant I had potentially infected every tank. I very rarely use feeder fish, and I have allowed some of the livebearers in my brackish tank to breed so I have my own source in the future, but this time I wasn’t as lucky as I had been every other time.
I did a lot of internet searching and I found a huge amount of people saying you could not use sea salt. It seems most of these people had little or no understanding about Ich and the effect of salts. Of course even fewer know the difference between the different salts a person can obtain.
The active ingredient which kills Ich, is the salt itself. Everything from table salt to marine salt is just as effective at altering osmotic pressure, the problems stem from what is added/present in the salts you buy. Many table salts will have anti-caking agents which could harm fish, marine salt contains minerals which will buffer your tank and raise pH, for example.
Another part of the research which really surprised me was the measurements people were giving, measurements that really were not appropriate and actually would give you more than three times more salt that I used and six times more than actually required.
What I did:
Day 1. I did a 50% water change on each freshwater tank and raised tank temperatures in my freshwater tanks to 90°f (yes, 90). That is 32.2°c. I did this immediately even though most advice is to do it gradually. I did not raise the temperature in my brackish tank because the salinity in that tank is always capable of killing freshwater Ich. The rise in temperature is to speed up the life cycle of the Ich so the salt could take effect quicker, and at 90°f I believed it quite likely I’d kill the parasite with temperature if not with salt. I simply observed the fish for signs of stress.
Day 1. I added one flat (not heaped) teaspoon of Instant Ocean Aquarium Salt per gallon, but only to half the tank. ie: For my 55 gallon tank I added 27 teaspoons. Do not add salt directly to the tank, get a small bucket and fill it with water from the tank, add the salt you want to add to it, stir it a lot until you can’t feel granules of salt anymore with your hand and pour it back into the tank. I recommend adding it into the stream of water from a filter so it gets dispersed quicker.
Day 2. The next day I did the same again. So now I had 55 teaspoons TOTAL in my 55 gallon, 75 in my 75, etc. This gave me a salinity of 1.005 (1.002 is what most seem to believe kills Ich).
Day 7. I waited about a week. I hadn’t had any signs of Ich since day two. I did a 50% water change (make sure you try to match the temp of the water going into the tank with the water coming out) and gravel vacuum. The fish in all tanks had shown no signs of stress due to temperature or salt. I lowered the temperature to 85°f. Because of the water change, the salnity was now 1.0025.
Day 8: Lowered temp to 76°f. If the fish were showing signs of Ich later than day two I probably would have waited longer.
Day 15: After waiting another week, did another 50% water change and gravel vacuum. Tank now had 1.001 salinity. If the fish were showing signs of Ich later than day two I probably would have waited longer to do the water change.
Day 21: Another week later and the tank is now basically back to normal. There is traces of salt which will remain but after another weekly water change the salinity now reads as zero.
Note: Warmer water means less oxygenated water. Salted water means less oxygenated water. It is therefore very important than when treating to make sure you have airstones supplying oxygen into the water.
Your mileage may vary, but I wanted to put some kind of reference out there on the internet that using sea salt for marine aquariums is an option for freshwater Ich, infact, if you have high pH and hard water fish, I’d probably recommend it! My Silver Dollars, who are accustomed to my high pH tap water) have spawned since this recent outbreak and a month later I have seen no further signs of Ich in any tank.
Things which annoyed me on the internet:
People who said marine salt wasn’t an option.
People who told novices to use three TABLESPOONS per gallon.
People who say shrimp cannot get Ich (see photo below).