Use the permanent marker to draw a circle around the circumference of the soda bottle between where the bottle begins to narrow inward toward the mouth of the bottle and the mouth itself. This should provide an opening large enough to put your hands through so you can organize the aquarium.
Lay the bottle on its side and carefully use a serrated knife to cut the top off the bottle along the circumference drawn in the previous step.
Pour about 2 cups of the aquarium gravel into the base of the soda bottle.
Plant the aquatic plant in the gravel by placing it on top of the gravel and then adding an additional 1 cup of gravel over the roots of the plant.
Add tap water into the bottle until it is filled about 3 inches from the newly cut top of the bottle. Allow the water to settle at room temperature in a location where it receives indirect sunlight for about a day.
Add the two guppies and the water snail into the water.
Feed the guppies according to the instructions on the label of the fish food. The snail will eat the algae that forms in the environment from the fish waste and the plant will survive on the nitrogen provided by the fish waste and the sunlight.
Place a sheet of plastic wrap over the opening of the aquarium and carefully poke holes in the plastic wrap with a pencil to allow for air flow.
Use the rubber band to secure the plastic wrap in place. This way the plastic wrap can be removed when it is time to feed the fish again and replaced once they are fed.
Remove half of the water from the environment once a week with a small cup to prevent overgrowth of algae. Replace the water with fresh water, but allow the water you are adding to the environment to set for a few hours to a day so it can acclimate to the temperature of the room and cause less shock for the fish once it is added.