Henry, a few weeks ago I tuned a cheap chinese melodica using the “drop of nail polish on the reed” trick. It works! In fact, I found it much simpler because if the tuning gets too low, just clean the reed with acetone (you can clean a small part of the drop too). However, I suspect that with use, the drop may come off the reed, which in the long run would cause a greater number of tuning processes. In the first days the nail polish left a considerable smell, but then the smell was gone. I think that something to consider is how toxic it can be to use this product on the melodica. Reading about the chemicals used in nail polish, I was startled. Either way, there are less toxic or “green” nail polish models.
I was curious about your testing tube for tuning. What’s it like? Could you share a photo? Thank you!