Diflubenzurone is that the name of the medicine? I could find something this name being sold in Germany.
The smallest is a pack of 1 liter (0,264 gallon), enough for the treatment of 6600 gallons
(for koi-ponds) !!! Yeah! The tank to be treatet contains about 37 gallons. So I can use it ... let me calculate .. about 178 times. If I use it every month once, it will last almost 15 years....
Nevertheless, thank you very much for this advice!
I will think about it, probably I will try it.