Posted by Scott Willbanks to All About Pond Pumps
Direct drive pumps for ponds originally came from sewage pumps. This is why many open bottom direct drives are able to handle solids, some can take in solids up to 1 ¼”. Some are not capable of handling solids, and then they usually will have a screened bottom to keep debris from getting inside the submersible pump. The only problem with these is maintenance increases as they will need to be checked for small twigs, leaves and other debris attaching itself to the screened pre-filter. The open bottom pumps are best placed inside a skimmer so it will protect very small fish, frogs etc. from getting blended through the impeller.

The impeller is attached to the motor through a drive shaft.  Seals are on the drive shaft to keep the water out from the electrical components inside the pump. When a pump fails the most common problem is seal failure and will trip the gfi. This is why some manufactures have now used two seals. Lubricating the seals will help with the life expectancy. Other problems include a loud continuous noise caused by bearing failure. Most direct drive pumps have a 2 year warranty; some have even lasted up to 15 years such as the Tsurumi brand. Usually the pumps are filled with non-toxic oil that will not harm the environment of the pond if it leaks. A direct drive pump should have some head pressure against it for a more efficient and longer lasting pump.
Example: If running only 4' of head the pump will run harder and less efficient than at 7' of head. Putting a ball valve after the pump and closing the valve some will help keep your pump lasting longer.

In most cases they are more expensive than mag or asynchronous pumps. This is due to the amount of water it can push vertically and or horizontally for a long distance. Most have a medium to high head height pressure that can power many applications such as large waterfalls, a pressurized filter system and many more applications. A few are available with a lower head. They are also known to be quite heavy.