Does anyone else get what looks like a light oil-slick on the surface of their water? I usually wash my hands/arms with dish soap before I stick them in the tank – especially if pruning or water changing. This stuff seems to appear on its own after a period of time (i.e. after a couple weeks).

I don't get that problem, but I also have an overflow that effectively skims any surface slick off the water, and I use a continuous dripping water change.

You know I just had a thought. It might be from the stuff I feed my fish. I feed spectrum pellet food in conjunction with a flake food and the spectrum is 45% protein. That's probably what does it.

I get it off of driftwood I have in some of my tanks. I have a huge piece in my 75 that continues to give it off. When it was new, I would periodically take paper towels and lightly lay on the water surface and it would soak it up..witih water too, lol. Took several paper towels with the 75 gallon tank, and had to be repeated. I've had the driftwood a year and is not near as bad now.

I tried the paper towel thing once and hated it. Now I syphon it out with the water change.

If you drive the O2 rates up through plant growth and good CO2, the surface films will go away.

I have identical tanks in terms of dosing, substrate plant biomass etc, when I varied the CO2 and reduced it, I got the surface film. I've repeated this on several tanks over of the years.

Better plant growth= less film.
These are for non surface skimmed tanks and with flourite, ADA AS, and Onyx sand.

With and without driftwood.

Regards,
Tom Barr