Warning for using filters with LT55

I have noticed a big difference in image quality with the DMW-LT55 and my cheap UV-filter from Roxcore..Sure, I shouldnt be surprised but I have used this filter on my Nikon D300 with good optics and didn´t notice any big difference..I use to have Hoyas or B+W filters but for some reason I bought the Roxcore (not sure the brand is available worldwide, but there should be similar cheap filters) I got better results with the i-zoom setting than with the LT55 wich seemed strange, so I took the filter off and it was a big difference, both with and without the teleconverter but I think the difference is bigger with it, probably due to internal reflections inside the LT-55´s attachment tube. I hope its because of the filter being bad, so I can use a filter as long its a good quality one...Its nice to be able to have a filter for protection,

the attached photos, first time I post pictures here, hope it works. Check them out in 100%, I have cropped them from original jpegs, nothing else.