Does a biological media fish pond filter (without any mechanical media) really keep a fish pond clean?
I know that they keep the water healthy but I am having trouble believing that it will filter out small enough particles to keep it really clean. I have one in mind to buy that has a built in ultraviolet light. I don't mind spending the extra money because the ease of maintenance and the idea of not having to disassemble and clean mechanical filter pads is very appealing. But, I do want very clean water. Any help would be very much appreciated.

You can use a powder called "pond zyme plus". This helps break down the tiny particles and will give you clear water, however there is nothing like a polishing pad to give you absolutely clear water. I have tried, over many years, home made and retail filters.

The most important thing to do is oversize your filter and control the population of you pond. My ponds are crowded so it takes more maintenance, I'm retired so I enjoy the work.