In each two liter soda bottle container we place hydroton
balls (clay growing medium for hydroponics) and add
suitable vegetables and herbs such as the curry, and
leafy green lettuce growing in this photo.
As the waste laden water enters the hydroton balls the
"nitrogen cycle'' is allowed to take its course, establishing
a colony of nitrifying and denitrifying bacteria.
The nitrification cycle is a process that converts the toxic ammonia in fish waste into other less harmful nitrogen compounds that are then absorbed by the plants as fertilizer.
Many species of bacteria such as nitrosomonas and nitrobacter do the conversion. Some species convert ammonia (NH3) to nitrite (N02-), while others convert nitrite to nitrate (NO3-). As the tank cycles, natural bacterial colonies are established both in the filter bed and in the hydroton balls that convert ammonia -> nitrite -> nitrate.
At this stage the plants can directly absorb the nitrates as fertilizer leaving the water cleaner for the fish and providing healthier vegetables for humans to eat.