I was reading about this filter, which I came across on EBay and was going to purchase it but I have no idea how it works. Do you have any info on it?
Thanks for your letter and the photos! That appears to be one of the original French Undergravel Filters, which appeared in the early 1950's and which began the undergravel filter (UGF) timeframe in aquarium history. In fact, I had the first one in the US in 1950! As a keepsake it might have some value, but as a filter, it's quite worthless in this day and age.
Below is some text from a future article I'm writing and you might find it interesting.
"In 1950 a guest speaker from France visited the society and described a device called the 'French Invisible Filter.' It was nothing more than a foot long porous tube with a chamber on one end holding an airstone. He said if it were buried in the gravel the upward flow of air bubbles would cause water to be drawn into its body from the surrounding gravel and this would encourage more efficient bacteria processes in the gravel. He was kind enough to offer one of the two 'filters' he had with him for the evening auction and when that time came, I was the only bidder! If memory serves me correctly, I bid .25 cents, my whole weekly allowance!
The following morning it was installed in my aquarium a short distance from the front glass and hooked up to an air pump. A few weeks later the dark line of detritus just below the surface of the gravel appeared to be far less than when installed. I was impressed, but did not really understand the reason for this improvement. At the next meeting I was asked to come up on the stage and tell everyone about what I did with that odd device. I explained how it was placed and the result seen, and was laughed at by all and told I was imagining things, as water cannot be drawn through gravel! From that moment on, I made up my mind to better understand what was really happening, not only in the gravel, but other physical relationships in the aquarium. And furthermore, to this day I believed this 'first' undergravel filter (UGF) used in the United States actually showed some positive results, however, they were no doubt quite marginal."
If you have any other questions, please contact me.