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RE: Surface scum

> Date: Tue, 25 Nov 1997 16:34:53 -0700
> From: Charley Bay <charleyb at cytomation_com>
> I'm going to go out on a limb and speculate on two primary answers:
> (1) greasy/fatty fish foods
> (2) high nutrient/biological load that encourages relatively higher
> bacteria populations.
> I've been toying with my tanks quite a bit on the surface scum issue.
> My 180g has a low fish load (20" fish, barbs/tetras) and no fertilization
> or CO2.  I surface skim, and never have scum problems.  If I turn the
> skimmer off (have absolutely no circulation or filtration whatsoever), 
> I will develop a slight layer of scum over about a week if I'm feeding
> fatty foods, like freeze-dried brine shrimp (it seems to be the worst).  
> I'm probably a horrible master, but I've actually rotated a solid week 
> of single-variety food just to see what builds up surface scum.  Some 
> of the cichlid or community pellets or micro-pellets don't appear to 
> generate any or as much surface scum after a week or more. (I'm
> next going to move onto different brands).

If you want a real outlier in this study, try Vibragro.  It's touted as
something to really get little fish to grow fast.  The fine print in the
enclosed booklet, that you don't read until you've opened the can, shows
a very high fat content.  It causes a nice rainbow greasy film effect.

> Of course, I'm interviewing my fish regularly to see how they like
> their diet and excercise regimen.  I have no mortality, though,
> so I guess it's "adequate".

My fish were dubious about the flavour and mouth feel of Vibragro, but
they seem to be getting to like it now.  After the on sale trial can is
gone, I don't plan to buy more.  Can't you just hear the little critters
yammering for their junk food.  (Actually, high fat diets are commonly
used for growing fish in aquaculture - it is good for them when they are

I'm glad you are doing Actual Science (or reasonable facsimile), and I
hope you will report the results to the list!

> As an aside, I'm thinking about starting to sample my tanks to
> identify quantity/variety of active bacterial populations as it relates
> to surface scum.  I work at a company that makes very expensive
> instruments that sort cells, and we can just run the samples
> through.  This "flow cytometry" is apparently in widespread use
> analyzing zooplankton/phytoplankton or other critters in marine
> systems... it seems like we freshwater guys should be able to use
> it too.  You should *see* the amazing stuff they are doing with
> yeast cells and ... beer.

Bacterial cells are much smaller than eucaryotic cells,  but it would
be interesting if your equipment could manage it.