My pond has a Hozelock Bioforce Revolutions filter with a matching Ciprio pump installed about 8 years ago. The water from the underwater pump goes out through 40mm hose, exits the pond up a rise to where the filter is mostly buried. The top of the filter is about 2' above the water level. The 40mm hose from the filter re-enters the pond on the plant shelf about 6" under the surface, generating a visible current that the fish can be seen swimming against. Over the past few months there have been quite large amounts of air periodically coming out of the outflow pipe, and bubbling to the surface. There are no water leaks, and no loss of water from the pond. Both ends are underwater, so the air must be getting in via the filter. I can't work out why, or how, the air is getting drawn in. I would have thought that the water pressure would prevent this. Plus, if air can get in, why is no water getting out? I normally just use the Revolutions handle and waste pipe the keep the system clean, but I have just taken the filter apart for a more thorough clean, though it was surpringly clean anyway. Can anyone explain this to me please?