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Dark green algae/diatom bloom on wood



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Oct 22, 2016
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The film on the surface is most usually a protein film, more common in planted tanks. It is, again, related to organics. Over my 20+ years, I have had it only in a very few tanks, never in the others. I used to use the surface skimmer attachment for canister filters (I had Eheim) but aside from plant bits getting pulled in and clogging it, very small fish also got sucked in and they usually couldn't be rescued; otos were fond of getting themselves stuck. So I took off the skimmers.

The protein film is most likely to appear along with cyanobacteria or algae problems, which makes sense since they are all caused by the same thing, organics. Low oxygen will cause problems but not directly cyano. I have increased the surface disturbance just a tad to increase oxygen which I believe was minimal in the early morning after a night of CO2 building up naturally. I had no algae or cyano issues, but the increased surfacing of the corys alerted me to this, and the increased surface disturbance solved it. It doesn't take much, just some, and this was most likely due to the heavy planting in this tank. My smaller tanks, 40g and under, all have simple sponge filters and they have no issues with oxygen, CO2, algae, cyano...whatever.

So back to the film, I now see it some weeks but only in my 90g which has an organics issue that I cannot track down. I deal with the film by turning the water changing unit upside down under water and pulling in the surface water. Takes a bit of time, but it does work. I tried the paper towel on the surface method and it was a real mess and ineffective anyway.
Loving the idea of reversing the syphon to pull water from above. I'm sure it's tricky, but frankly I've tried skimming with a jug, turkey basters and every brand of paper towel trying to find an effective method to clear the film. Only constant agitation (and once a drop of melafix as the oil in it seems to split across the surface), has ever worked for me.