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Hello, i've noticed the last couple of times I've done water changes that the surface of my tank appears to have a thin layer of what appears to be an oily substance.
You can only see it when looking up towards the light.

Any clue what it is and how I can get rid of it?
 

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Yea I have that too. Any oily substances that get into the water from anything you put into the tank like your hands, rocks, etc. (but most likely your hands) will eventually make its way to the surface of the water and stay there. Any type of oily substance and water do not mix, and since oil is less dense that water, the oil will all reside in a layer on top of the water. What I did to get rid of it was use a bucket (obviously rinsed inside and out thoroughly to make sure I wasn't adding more oils to the tank) and tried to do part of my water change from the top of the tank. A siphon directed to the top, near the oil layer, may work too, but I kept losing the siphon every time I tried to get close enough to the oil layer to remove it. All in all, I think this is a common problem in aquariums in general, so it should not cause too much of a problem, but someone else with more experience will have to tell you the ramifications of this.
 

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This is called surface scum. It is a build up of organic compounds in your tank, oils included. How often are you doing water changes? It can come from decaying plant material, over feeding, etc. The best way to help combat this is to direct your filtration output so that you get a little bit of surface agitation, making sure you get any dead plant material out before it decays and doing regular water changes. I do 50% weekly.
 

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Mollies will skim the surface for this stuff and clean it up pretty quickly. (That and the fact they'll eat BBA are the only reasons I have mollies in my aquarium). You can also try a surface skimmer, but that's just more $ to spend.

If you don't like mollies, the best way to keep it down is what's already been mentioned: don't overfeed, remove decaying plant material, and keep up with water changes.
 

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I had this problem once and in my case it was from a piece of driftwood that was rotting on the bottom side. I removed the wood and within a few days the problem was gone.
 

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davemonkey, what type of mollies do you keep that eats BBA and how effective are they? How many do you keep in what size tank? Sounds interesting enough to try.
They are black mollies (black sailfin?). I have 4 in a 50 gal and they do a good job at general cleaning. The first BBA I saw them eat was off my driftwood, then they hit the plants. I had 5 SAE's, but re-homed them after they started eating my mosses. I have noticed a slight re-currence of BBA since getting rid of the SAE's, but the mollies keep it pretty well under control.

The trick is to keep them hungry for it (and they eat ALL types of algae for me) but not so hungry that they'll eat your plants. I feed my fish once a day (lightly), and go a day without food now and then.

The mollies kinda throw off the general "schooling tetras" feel of a planted tank, but to me it's worth it, especially since they don't eat my moss like the SAE's did.

Edit: I should note, the article I read that made me decide to try mollies said that any molly would work, but especially the Sailfin and Black Mollies. I don't remember where the article was, but I'll hunt for it and post it when I find it.

-Dave
 

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Jeff,

The surface skimmer will help to eliminate it but will do nothing to fix the cause of the problem. Surface skimmers work really well in saltwater because the organics go to the surface but don't stay in the water column as much as they do in freshwater. It is more important to get to the bottom of the problem.
 

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Jeff,

The surface skimmer will help to eliminate it but will do nothing to fix the cause of the problem. Surface skimmers work really well in saltwater because the organics go to the surface but don't stay in the water column as much as they do in freshwater. It is more important to get to the bottom of the problem.
:rolleyes: DUH!!
 

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What i do is take a sheet of paper towel/napkin and lay it flat on the surface and pick it back up,do this several times with a clean paper...don't use the same one for obvious reasons..works like a charm
 

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Mollies do a great job removing surface scum but they need at least a little salt in the water and a pH of 7.0 or above to do well and stay healthy. They prefer hard to soft water and can do well in saltwater tanks. Also, there should be more females than males. They eat algae non-stop and deposit a corresponding amount of waste. It may be a good idea to read up about them before purchasing some. You might find some of the other ideas posted work well for you (some people even use plain white paper towels to soak up surface scum) and that adding mollies isn't necessary.
 
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