There are a number of non-toxic options for managing fountains and sludge, depending on what the sludge is.
If the sludge is composed of aquatic plants or algae mats, then there is a great section in the following extension publication on aquatic plants and how to control them. Methods such as adding a bubbler, restricting fertilizer use for properties adjacent to the pond, and occasionally drawing down the pond are all methods suggested. The section is chapter 10 in the following document that can give you several tips.
Feel free to send me an email to get some specifics from that chapter. I also have free copies in my office available.