The fishpoo in the water supplies the plants with nutrients. The bacteria in the roots of the plants cleans the water, the action of the water falling down thru the bottles aerates the water. The whole thing is synergistic like permaculture. Once its set up, monitoring and harvesting is all that is necessary. This protected the pump from getting gummed up with whatever is in the water.
Algae gunks up everything.
It grows where there is still water and sunlight. To prevent the algae from growing in the water containment area, I have found that hay in a mesh bag, floating in the water, somehow stops the algae. Timing and intuition are important. Once you are ready to place your columns of 2 liter plastic bottles with the seedlings, you are ready to add the fish. How many, what size, and what kind you may wonder.