aeration

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...
actually, the smaller the air bubbles coming out of the air stone the larger
the over
all surface to volume ratio. which is why airstones can put so much more air
into
water that just disturbing the surface.
now if those bubbles break the surface and rise into the air there is exchange
both
on the inside and outside of that film of water.
an airstone in the water moves a water column up to the top and it flows
outwards
creating a moving column of water that brings water from the bottom up to the
top or
vertically. this is much better circulation than a venturi that moves water
only
horizontally across the surface.

In a pond, as opposed to an aquarium, in order to get an equivalent water flow
and gas exchange from an airstone would require vastly more air than most people
are willing to spend the money on, in my opinion. And the water movement into
the column disipates over a fairly short distance. oxygenating fresh water is
simpler, and less critical than it is with salt water, especially with goldfish,
since they can practically live in sewage. Larger fish like koi are a different
story. But the whole issue is probably moot anyway, since if the fish aren't
suffering, there is really no point in going to the added expense and trouble.
I can see a situation in which you have too much bioload in the pond then you
would need to add air, but to me the best solution is to simply reduce the
bioload. Fish gasping for air because of too little oxygen in the water is a
sure sign of a pond that simply has too much growth. The only time I use an
airstone is in the dead of winter to keep the water moving, since I don't want
to damage my pump by having it freeze up on me. I do use a floating de-icer,
but that doesn't guarantee that the water in the filter won't freeze.

pumps should not be put on the bottom of ponds. too much chance of something
going
wrong and the pond being drained.
Ingrid

I agree that you should never place the pump directly on the bottom of the pond
unprotected. However, if the piping is installed properly, and you add an
adequate pre-filter to the pump intake (one that can be easily cleaned), I see
no reason not to put a pump on the bottom. I don't do it simply because my
filtration system is designed differently. Personally, I use the suction method
in my filtration system instead of positive pressure for several reasons.
First, I have a 5 gallon pre-filter in the bottom of the pond, to which is
attached to the intake of the main filter. The main filter consists of an old
jacuzzi swimming pool filter housing (about 40 gallons capacity) that I have
adapted for use in the pond. The filter is submersed in the pond out of sight
beneath my waterfall. The intake line attaches to a 90degree elbow, which is
attached to a threaded coupling that goes into the bottom of the filter tank.
Attached to the coupling on the inside of the tank is a stainless steel basket
line strainer that catches any debris that happens to get past the prefilter
(which usually is very little). Surrounding the strainer is about 50 lbs of
quartz aquarium gravel, which fills the tank about half way to the top. Above
the gravel is a very course mesh polymer pad to keep the gravel from getting
sucked up into the pump. The pump sits about two inches above this pad. The
pump is attached to the top plate via fittings, into which the outlet is
attached. The electrical cable also goes through the top plate via a rubber
electrical grommet. The top of the filter sits about three inches below the top
of the water in the pond, so the pump intake itself is submerged into about the
top 15 inches of water.

The effect is just to suck or pull the water through the system. The only water
going through the pump is filtered water. It is at the end instead of at the
beginning of the filtration process. An added benfit is that it can never pump
all of the water out of the pond. I always know when the pre-filter needs
cleaning or the water level has dropped for some reason, because the pump will
raise pitch due to the water level being drawn down to the level of the pump
intake (inside the filter housing), causing a slight cavitation. Since I check
it every day, there is very little risk of the pump burning out due to extended
caviation and/or overheating. And the pump was actually designed to run
practially dry. When I get a chance, I will give you a link to some photos of
the filtration system. Oh, and because I use a very efficient pre-filter, I
almost never have to backwash the main filter, which allows the bacteria to
flourish undisturbed. I do, about once every six months, open the filter and
swish the gravel around to get rid of any potential clumping, and then pump the
gunk out into a bucket. But I haven't had to do it yet this year. I still have
full water flow. One day, when I decide to spend the money, which will probably
be whenever the pump fails, this is the pump that is on my "some day" wish list:

http://www.flotecpump.com/

Actually, the air water interface is at least as important, if not more
important. The larger the surface area of the air/water interface, the
greater
the exchange of gases. While an airstone can pump air through the water, it
does so in a limited way, since it is only making contact with a small area of
water at a time and only influences the immediate area around the airstone.
By
inducing a current across the surface of the water with a pump located at the
bottom of the pond, you will induce more gas exchange because of the greater
surface area involved. It also helps in preventing zonation from froming in
the
water column, since you are pumping the water from the bottom and streaming it
across the surface, which in turn, allows oxygenated water to move towards the
bottom of the pond. The ripples going across the surface of the pond also
looks
nice.




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