The first two forms of nitrogen in abundance are unhealthy in the pond. There are some submerged filters that claim to be biological, but they act largely as mechanical filters, just trapping debris, and of course some bacterial activity will occur, but the external types are most efficient. You can build your own biological filter in a wine barrel, or a series of barrels, water passing from one to another, then back into the pond. When planted with aquatic plants, it utilizes mechanical, biological and botanical filtration. There are some model sewage treatment plants within California that have utilized this water purification method on a grand scale with remarkable results.
Pumps are either submersible or dry.
For most small ponds, a submersible pump is sufficient, a prefilter can be attached to the pump to prevent the pump from fouling. With very large ponds, and external dry pump in a separate housing is the best way to go for easy maintenance. There are dozens of brands on the market. Many are oil filled, which can cause problems in the pond if they should leak. Epoxy encapsulated pumps are safer for aquatic life as they won't leak.