So, now it has happened to me to. Algae has started to grow in my windowfarm, or rather in the tubes. I knew it would eventually start to grow algae because the tubes are transparent, but that was what my local pet store had. The plants doesn´t seem to have suffered.
Now I have washed all the tubes with very warm water and soap and re-installed them. To reduce the possibility that it will come back, I covered the tubes with white tape.
