The stringy poop is probably caused by intestinal problems. There are several medications for treating this. Although, I wouldn't worry about those quite yet, as ich will definitely kill the fish and much faster. Don't combine treatments.
It will be hard to test for the cupramine if you do a water change before the 2 week period is over. However, by taking into account the amount you take out you should be able to determine tha amount to re-add to get it to a certain ppm. You should have added 1.5ml of cupramine to 15g(30g = 3ml of cupramine), so I assume you added a total of 3ml on day one, right? If you add 3ml's more on day 3, then your total will be 6ml cupramine. A 50% water change will result in a 50% reduction of cupramine, so add 3ml back to the tank after adding the new sw.
Personally, I treated 9 fish in a 42g without changing the water until after the 2 weeks was up. I then changed 50% after 3 weeks more weeks until the 6 week period was up and had no problems. All the fish made it through the treatment. That was 4 years ago. I still have most of these fish.