How big was the tank and how "clean" was it?
I quite often hear people remark on them eating prepared foods but of the two I've kept and the experience of some others, I have yet to see them do so first hand. I would call those that do the exception, not the norm. They are for the most part slime/filamented algae eaters and scrape their food off surfaces in the tank. Larger tank obviously having more to offer than smaller tanks, the smaller not usually being able to sustain their diet long term. Fish will typically have to "learn" to feed from what you give them but many do not or will not. I place the Lawnmower in the category of most likely will not.