That certainly wouldnt be a fresh water dip, it would be more of a low salinity dip. If you're going to do a FW dip, do it as a FW dip so you get all the max benefits FW provides.
For a FW dipping, what I do is get a 5g bucket or a 10g tank, whichever you prefer. Then put a heater/powerhead in it and get the temperature equalized. You can also use a airstone instead of a powerhead, just depends on your tank size and powerhead gph. I have a really low GPH powerhead I use with a small venturi ability, so it does good for keeping the water aerated.
Anyway, get the temperature correct, and then put in baking soda. You can dump the stuff in and never get above 8.2 pH or so. But I basically put in a tablespoon per 2 gallons and every time I've tested, the pH is 8.2, so I dont even bother testing pH anymore. Then you just dose as needed, depending on what you're fighting.
I tend to use methylene blue as it actually helps oxygen processes on a cellular level. Not cellular as in AT&T, but as in the hemoglobin in the blood is actually affected and helps transfer oxygen faster.
And I'm a big fan of formalin, for a FW dip do 2.7 ounces per gallon of water, for 5 or 10 minutes. For a large angel I'd probably keep him in there 10 minutes.
And then please dont put him right back into a display tank. Put him into a quarantine tank and treat with copper for the manufacturer's recommended time length. He should be ich free and ready to go back into the display tank within a month.