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Mar 17, 2022
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After returning to the hobby I set up a single tank 25 gal cube, and used a lot of live plants and Co2. Early on, within the first month or two, I had bio-film on the water surface, but after adding some fish and with the tank maturing it didn't last very long. Maybe a week or two max.

I am hoping to add a few new fish to the main tank, so I got carried away and started a 10gal quarantine tank (qt). I put the canister filter for the qt on my main tank for about 3 weeks, and then put it on the qt for the long haul. After another month I added some simple plants, some watersprite, an amazon sword, and some monte carlo. With no fish in the tank, I have had an oily film on top of the water for a few months now. I change the water almost every other day, and try to siphon from the top of the tank, but the film just seems to persist.

I am at a point where I am ready to buy some new fish and use the qt, but this film has me slightly uneasy. So I guess I am wondering ... is this normal? and also, do some plants like watersprite, which seem to be growing like mad, tend to leave more of a film than other plants?

I was using the API water treatment with aloe for a while (stresscoat), but then started reading on this site, that it may not be great for fish. Either way I switched to api tap water conditioner about 2-3 weeks ago. I doubt the api stress coat is to blame here as I was using it on my main tank as well, and it never seemed to cause a film there.

So in summary, two tanks, slightly different plants, one with fish, one without. The fishless one continues to have any oily film problem and the other does not. Both use fluval canister filters, the same types of filter media, same gravel, etc. Maybe the only other difference is that i use an airstone on the main tank when the lights are off.

Any thoughts?

One other thing, I tried using the fluval attachment for skimming off the surface, but it was really finicky and oversized for the qt. I guess I could give it another shot, but it really seemed to be a piece of junk with rather inflexible hosing and just an ugly appearance.


Supporting Member
Tank of the Month!
Feb 25, 2009
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The surface film is probably a protein film. This is more common with planted tanks than non-plant tanks, but not always by any means. In my 30 or so years with several fish tanks I ran through two periods when this was noticeable, in a couple of tanks but not in others. The only problem is that it can thicken to the extent that it impedes the necessary gas exchange (O and CO2) at the surface.

I tried the paper towel trick, no luck. So during water changes I upended the water changer and siphoned the surface water (air and water, it is tedious but very effective). After a few weeks it disappeared.

On an earlier occasion I bought a surface skimmer attachment for the canister filter. It was very effective at removing the protein film, but more than once a small fish (otos seemed prone to this) got sucked in and not being able to swim in reverse, they died. I disconnected the skimmers.

Good move on the water conditioner. Aloe vera has no place in a fish tank, and it is known to damage gills over time. I know, some diehards will say it has to be over-dosed, etc, etc--but if it is not doing anything beneficial, and it most assuredly does not benefit, there isno need to be using a risky product. The API Tap Water Conditioner is about the best, most effective, and safest conditioner available. It is hard to say, but the Aloe Vera is a plant sap, and it may have partially contributed to the protein film, but I wouldn't press the point. Best not used regardless.

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