The general consensus of these under gravel filters is that they work quite
well. Having said that, like their aquarium counterparts, they require
periodic cleaning and that's where the trouble is. I can only imagine what
trying to clean one of these things would be like. Relocating fish and
plants, draining the water, cleaning tons and tons of gravel (depending on
the size of the pond), then putting the filter system back together, filling
with water and finally replacing plants and fish, all the while trying not
to damage the liner. Not that it couldn't be done, It's just that I prefer
my filter out of the water where it's quite easy to clean and maintain. Most
of those in this ng that comment on gravel on the bottom of their ponds have
either decided to refrain from using gravel, or have done so and regretted
it. I, personally, don't have gravel on the bottom of my pond. I came close
to adding it when the pond was new, to make it look more 'natural', but
didn't after reading many posts in this ng. Now I'm glad I didn't. The pond
bottom looks just fine after maturing some. One really has to look hard to
see the liner. Plants and a 'fuzz' of algae have it covered quite nicely.
Nothing wrong with the undergravel filters, they just take a LOT of work.
That's why some of the ones that you observed were 'nasty'. The owners
probably weren't maintaining them and they were no longer working.
Just my $0.02 worth, hope it helps you. Keep reading this ng, I'm sure
others will comment too.

Paul


"Barry" wrote in message
hlink.net...
Lots of folks in S. FL seem to like designing ponds with filtering systems
similar to aquarium undergravel filters. They run pvc tubes with small
holes
connected to
there pumps, which then transfer the water to canister type filters. The
pvc
piping is covered with a layer of gravel. I have seen many that are
crystal
clear,
while others are quite nasty. One aspect that seems good is that the water
is pulled
more evenly from all areas of the pond, as opposed to a drain on one end.

Any thoughts on using these gravel filters vs. smooth bottomed ponds
(vinyl
or sealed concrete)?
My pond is concrete with a black diamondbrite finish. I had once added a
layer of river rock
for contrast, but removed it due to the fast algea growth that occured.