Is the frosting coming from cold inside the line or outside the line? If outside, you might try wrapping the line with a/c refrigerant line insulation. If inside, can you place the pump where it will pull slightly warmed air -- or perhaps both????????

The frosting is coming inside the line - slowly builds up and will eventually fill the tubing and the air pump will not be able to pump air at all into the airstone. This only happens when the outside temperature is bitter cold. If it is only mildly cold, then the tubing stays clear of frost. The pump is kept outside. I tried putting the pump inside the house (the pond is 2' from my patio door, but the tubing frosted up even faster if the pump was inside the house, as the warm air from the house was hitting the cold air outside.

The longer the tubing the more chance of moisture collecting in the tubing. If the pump is sitting on the ground it will pick up that moisture. I place the ceramic pot as close to the water as possible and then place an insulated box around the pot. There is a slit in the insulated box to admit air.This is probably overkill in zone 5.

If it got really bad I would probably put a hard duty lightbulb inside the pot. It wouldn't take many watts to keep it warm and toasty if the box was not too large.

The air coming out of the pump may have a slight amount of heat in it (the "work" done by the pump). This cools off rapidly once your line is much longer than about 10 feet or so -- it makes no difference what the length of the line is after that...temperatures will be pretty close to the surrounding environment. However, given what you've described it sounds as if your pump is sucking in air that is essentially saturated. Try elevating you pump off the ground...a foot or even better, two feet in height should make a significant difference in the amount of moisture pick-up. But, as you discovered, do not put the pump anywhere it is warmer or could possibly pick up moisture. The use of an inverted plastic pail or pot can create a problem if it is placed over the pump on the ground. Here's why: during the day with the sun shining on the pot the ground underneath the pot will warm up. At night as the temperatures fall outside, the ground is still a bit warmer underneath the pot with the result that moisture is given off at a slightly higher vapor pressure. Then as it is pumped out into the line it will cool, condense, and freeze. I've used a pump but I have it in a box made from exterior blue board (you can buy a 10 foot long x 2 feet wide section at Home Depot) which covers the pump and rests on another piece of blue board. This does not significantly heat during the day and more importantly contains little or no moisture to be sucked into the pump as things cool off at night. No problems in over 10 years. Or you could put your air pump on a large sheet of plastic to stop moisture coming out the ground near you pump. Just a thought. Hope this helps.
---David

Hi groomer,
I put my air pump under an upside-down clear plastic shoe box. The pump sits on a brick and there are more bricks on top of the shoe box to keep it in place if it's windy. The power extension cord and the plug are also on a brick under the shoebox, so nothing touches the ground and no moisture can get to it from above. Actually, the whole thing sits on the little bridge, directly over the opening between two boards, which provides nice cold and dry air. It's also very close to the water, and the two air tubes dive in immediately. The air stones are suspended from a floating de-icer which isn't on. It's just there to hang the air stone from.
Sometimes I plug the de-icers in when it's super cold but probably I wouldn't have to.

The whole thing works excellent. Until we have a power outage...

:) Mary

I use icemaker tubing works fine I live in Mass.

I have discovered over the years the problem comes from using clear plastic tubing. The sun will beat down on it causing condensation inside the line and them it freezes. 3 years ago I switched to a silicone rubber tubing that is black and no problems ever since.

Just read this and had a few thoughts-
I am using 2- 300W aquaruium heaters inside my filter. The water is heated as it passes through and is definately aiding on keeping the water a warmer temp. Another way to aerate besides using an airpump is to use a submersible pump with a venturi. The venturi pulls air in from the surface with an airline and really saturates the water well with oxygen. There is one at Wal-Mart for about $17, also called a powerhead that is used for aquariums. It has a venturi attachment and just uses tubing like the airpump. If the water is kept warm by means of a cover or even an aquarium heater or both, either tubing shouldn't freeze.