Candiru are known to enter human urethra (they mistake the urea for the metabolic waste given off by a fish's gills) but I've never heard of them making it to the bladder. Candiru are extremely thin little catfish with specialized fins that allow it to lodge itself inside you.
I don't think you could cram a 1.5 cm. wide Betta up a male urethra with a shoehorn and a tube of KY!