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RE: Surface scum

> "Frank I. Reiter" <FIR at istar_ca> wrote:
> [snip, surface scum]
> > 2) What does it's presence tell me about my tank?

> George Booth <booth at hpmtlgb1_lvld.hp.com> responded:
> It says "your tank is just like my tank(s)".  All of our tanks have
> had this surface film ever since
>   1) we started raising plants
>   b) we started adding CO2
>   iii) we started fertilizing
> All this was so long ago, I forget which might be the case <g> 

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).

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 guess is that the major factors are fish food type/volume, 
and many fish (with many slimy coats and much more biologically
active waste products).  For these experiments I've been working
with larger tanks and lower fish populations just to try food type
as it relates to scum production.  In writing this, though, I just 
realized I've done a bad thing by not measuring the quantities
and times I've added the food, though.  I will next move to constant 
food type and larger/smaller fish populations.  While I suppose plant
biomass volume may be a factor (plants do produce lipids and
fats of their own), I don't think it would be the lion's share (with
absolutely no data I'm guessing food and fish volume should
account for at least 80%).  

Over-fertilization, of course, would merely increase the range 
of biologically viable growth/reactions because you are moving 
ever-farther from a sterile or nutrient poor soup.  That would 
increase bacterial activity and surface scum, so I'm not fertilizing now.

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

I agree with others that surface skimming is easiest, followed by
water changes or other mechanical control (I saw paper-toweling of
the surface here).  I've even let my tanks "sit" for a couple days
prior to a water change, let the scum settle on the surface, and
used my python to "skim" the surface water, all of which is removed
in the water change.  That seems to work really well for me.

Perhaps I will eventually be able to recommend specific food types
or fish volumes, but my guess is that the fish still need that food
variety and really fatty/greasy foods are probably very good for
them (so this is probably a pretty moot issue that we should
just live with).  I might end up giving them the equivalent of fried 
chicken in gravy 10% of the time instead of 50% of the time, though.

Isn't it *love* that lets me force them to eat more heathfully than I do?

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.