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Oh, well I think the green already does look like liquid, and may not need much tweaking, once the rest of the image is like you want.
If it is a sealed glass tube, I don't understand the gray shading on top. I think instead, you might want white or white-ish highlights, and maybe even some reflections. You might look around for some tutorials on making things appear like glass (especially try those in my signature).
The part of the image that's white, inside the tube, above the green -- should that appear transparent, so that you would see the color of whatever background you have, seen through the tube there? (Perhaps it took on the white when you saved as PNG in order to upload it? If so, you should use File menu > Export Bitmap instead.)
I think you've made a great start, and it will be one step at a time. Plus I'm sure others will have some helpful comments as well. I'll be interested to see the final product!