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Re: QYNGA: Surface scum

> From: "Frank I. Reiter" <FIR at istar_ca>
> [snip]
> When the sum is thick I can agitate the water with my hand and it will
> break into small white bits.
> The questions then are:
> 1) What *is* this scum?

What we know so far about this stuff is that it is biological; it's some
kind of bacteria feeding off nutrients in the aquarium. I haven't seen
an identification beyond that on the APD. I have this intermittently on
my tanks.

> 2) What does it's presence tell me about my tank?

I get this most commonly when I have a tank with little or no surface
agitation. When there is a moderate surface flow as with a Whisper hang
on trickle style filter, the flow of water breaks the film up and it is
either eaten by the fish or drawn into the filter.  It has been
suggested that this is a kind of bacteria encouraged by the presence of
chelated iron in the water. It seems to occur under conditions even
where all nutrient concentrations are low or moderate. Low or high CO2
levels do not affect it; I've seen it on tanks with and without CO2.

> 3) How can I encourage it to go away and stay away?

Probably your best bet is to move and don't leave a forwarding address!
;-) Seriously though, I think the answer is in filtration. Either have a
filter which draws water from the surface as in an overflow style or
allow the return of an external filter to flow smoothly in a mild
waterfall fashion into the tank. To avoid excessive surface disturbance
and annoying trickling sounds, I glue a strip of thin plastic sheet onto
the outflow of my filter so that the water runs smoothly down it into
the tank. I suspect there is no effective way to eliminate the bacteria
themselves. At this time, we don't know what the bacteria is. It could
be introduced with fish, endemic in tap water or even airborne. I
suppose treatment with a UV sterilizing filter -might- be a possibility.
That's not very practical for most of us.

I think this QYNGA is because few of us have a definitive answer to
give. I look forward to reading what the others have to say about this
stuff. What's the technical term for it anyway? gunk? surface scum? ;-)