You might not be able to make it go much lower. Water hardness has a huge effect on pH. If you have hard water, your pH isn't going to move much. This is why it is important to know your local water parameters.
You really need a home test kit. The pet store's definition of "fine" isn't exactly what most fishkeepers call great. The API master kit is one of the best. Don't bother with the strips as they are notoriously inaccurate. Once you test your water then we can try to help you.
You actually don't need a bubble stone. If the tank is properly stocked enough oxygen from the air should diffuse into the water. The little ripple from the filter should be more than enough.
If you keep losing gourami, you should hold off on buying more until we can figure out what is wrong.