"Chris Barnes" wrote in message
...
George wrote:
That is true. However, few people build their ponds with an included
undergravel filter as is found in most aquariums. Most people who have
a biofilter (such as myself) build it as an enclosed structure or
vessel. Within that vessel, water is constantly being fed into it,
bringing with it oxygenated water. The sludge I was referring to occurs
on the bottom of the pond where detritus settles out and can become
anoxic in areas with little or no water flow, especially where there are
rocks piled up. If your biofilter is working properly, the nitrate and
ammonia levels will usually remain low, but the pH, over time will begin
to rise because of the build up of sludge on the bottom. That's my
understanding of what happens. It's a good argument for using
sludge-loving bacteria in your pond to prevent such build up from
occurring, and also argues for increasing water flow in areas where such
build up can occur.

I can see how that works for small, decorative ponds. But what about
large ponds (say, one that is 100'x40' and 20' deep)?


Large ponds and lakes are a bit different because of the dilution factor,
and because there are usually inflow and outflow routes. If they have
inflow and outflow routes, then the pH can fluctuate during times of heavy
rainfall, but under normal conditions is relatively stable. Having said
that, large ponds and lakes in temperate zones tend to stratify and then
overturn with the changing seasons. Large ponds and lakes in sub-tropical
to tropical zones rarely overturn, and so remain stratified for long
periods of time, which causes a buildup of gases in the lower levels,
particularly of CO2 and H2S. Disturbing those layers (say due to an
earthquake or landslide/slip) can cause a release of large quantities of
built up gases all at once, which can potentially be a dangerous situation
for lifeforms living nearby. But for the size of the pond you are talking
about, I don't think that is much of a problem. There may be some minor
stratification, but if it is in a temperate zone, it will overturn during
the spring and fall, and naturally get rid of those gases.

George