Water Surface Milky/oily layer

19 Dec 2007
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I've noticed that a milky/oil layer develops on the surface of my water -- that the overflow does not seem to suck up.

I also notice that this disappears with a water change but returns a few days afer.

Any clues on what this is and how to rid the tank of it?

feed once a day --- dry food
2 x per week frozen food

Tank specs:
450 litre
ATi 160 skimmer
carbon/rowa - 24/7
150w MH
4 Ati T5 39w (2 superblue/2white)
2 Tunze 6045 nano streams
Resun chiller
Aquamedic kalkwasser stirrer running with auto top-up unit
Ph 8.2
Salinity 1025
Calcium 300-350ppm
Phosphates -- dont really test
kH -- 10


Retired Moderator
29 May 2007
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Cape Town
Hi Raveman. The reason for this is a combination of things:
1) The water surface turbulence (the more surface water agitation, the better - so - either you must some power-heads explicitly pointing upwards towards the water surface, or have two reasonably strong power-heads pointing towards each other, that their flows converge at a certain point, which will force the water upwards creating water/surface agitation)
2) your over flow design - how much water do you actually get from the water surface that flows down into the over-flow? The idea of the over-flow is that it does "surface skimming", meaning that only the top +-1cm of water flows down the over-flow, to the sump, where the skimmer sits - that can skim out the dissolved proteins, that is causing the oily/milky layer on the water surface (surface film)
3) as Mekaeel pointed - you do not have power-heads that "forces" the surface water to the direction of the over-flow, that the surface film can be "sucked down" the over-flow
4) the frozen foods that you are feeding contains too much oils/problematic contaminents (are these commercially bought frozen cubes?)

Otherwise, you have a VERY decent skimmer.....

If you get these things right, then your issue will be resolved!