I did 25% water change every 2 - 3 days to keep ammonia down to safe level while treating ich with Cupramine or Copersafe. I also used one of those ammonia detection badges that stick on the inside. NH3 can rise very quickly and you have to watch it closely also using test kits to monitor daily. When you do water changes, you need to replace the Cu that was removed.
Using water from an "established tank" wil not do much as the good stuff is not in the water, but rather the rock and sand. It may be a good start, but watch you SA, especially with water changes.
Also, if treating with Cu, you MUST have a Cu copper test kit AND get the correct kind of test kit for the Cu you are using. If using a filter, remove any carbon while treating with Cu as it will interfere with the Cu levels.
Finally, IMO/IME, the only way to treat ich is hypo Sa or copper, all else is snake oil and a waste of money , time and fish!
There is a ton of info here on how to treat properly using Cu or Hypo, do some searching in the disease section and read, read, read.
I got it my first 6 months into the hobby, got everyone in a 30g HT and used Coppersafe; 3 of the 8 fish died, two of which were scaleless type that do not tolorate Cu well, the others are still alive today. Doing this is a big PITA, but its worth it to save your loved fishies!
If it is indeed ich, move quickly and don't be fooled if it goes away for a few days as it can come back with a vengeance!
Good luck!