Especially blue-green algae settle on the plants and everywhere else. The plants have become smaller and do not grow well any more. The only change in the aquarium we can imagine as a cause is that we replaced the fluorescent tube half a year ago.
Blue-green 'algae' (more precisely: cyanobacteria, because they are actually not real algae but rather bacteria that are able to perform biosynthesis) unfortunately may be rather obstinate.
Due to their relationship to (desired) bacteria it is best to try and push the competition equilibrium towards the desired bacteria. Unfortunately, there is no "patent solution" that helps equally well in all situations, since there are several thousand cyanobacteria species. There is everything from species that live in extremely pure water up to tohse that only occur in extremely polluted water.
It can often be observed in aquariums that cyanobacteria occur if the measured nitrate and phosphate pollutions are very low - which is usually considered an indicator for good and unpolluted water quality.
The following approaches are promising:
* Siphon off the layers at least once daily, as completely as possible. Do not get discouraged if a lasting success takes 2 - 3 weeks.
* Complete darkening for one or two weeks (e.g. with a dark blanket hung over the aquarium or black cardboard attached to the aquarium glass panes). Switching off the lighting itself is not sufficient, even daylight must be kept away from the tank! Important: First carry out a lerger partial water change of about 70 % (siphon off as much cyanobacteria as possible while doing so) and provide additional oxygen by good water agitation and additional aeration while you keep the tank dark! Then carry out another 70 % water change and return to normal aquarium maintenance.
* Changing the maintenance intervals, e.g. the water change rhythm.
* There are observations revealing that outcompetition is possible by directed addition of "desired" bacteria (sera bio nitrivec).
Dr. Bodo Schnell