A drop of inorganic fertilizer greatly enhances the growth of algae in a mini-aquarium.
The concentration of free-floating algae in pond water is sometimes quite low. Observation of pure pond water therefore does not always result in interesting microscopy. It is, however, possible to increase the concentrations of the algae by adding a small amount of fertilizer to the water sample.
Instead of adding the fertilizer directly to the pond water, I used some clean tap water. I scratched off some algae from the original jar and transferred it to the new jar. I also added one drop of liquid inorganic fertilizer, the one which is used to fertilize garden flowers. Organic fertilizer (such as Guano fertilizer) promotes the growth of bacteria and I therefore did not want to use it.
The first try was not successful, probably because I added too much fertilizer. The water formed a white precipitate which accumulated on the bottom of the jar, but green algae could not be observed. I repeated the experiment adding only a single drop for about 500ml of water and was much more successful.
I also supplied the jar with air, using and aquarium air pump to ensure sufficient carbon dioxide supply, but it also worked without air supply. As a matter of fact, some algae do not like it if the water is agitated too much. I placed the jar in a bright place to promote photosynthesis. Evaporated water was replaced every few days. After about 1-2 weeks the water turned increasingly green and a biofilm started to form on the glass surfaces. Bubbles (possibly oxygen) also started to appear on the surface of the water.
I found mostly single-celled algae and some ciliates. It would now be interesting to observe how a change in parameters (different types of fertilizer, lighting conditions, and water types) would promote the growth of different organisms.
Using soil-water as a nutrient solution
The type and quality of the water determines which algae can grow. Instead of using fertilizer, you can also dissolve some garden soil in water. Boil the soil-water mixture. This will extract nutrients and kill of microorganisms that might disturb the growth of the algae. Filter the mixture to remove the soil. Transfer some pond water into the (now cold) jar. You have to wait and be patient.