What you'll need...
1, 2Liter Plastic Bottle with Cap (you can add more if you want)
1inch thick piece of Wood/Glass/Hard Plastic (this will act as your aquarium lid)
Aquarium Silicon (or adhesive of your choosing)
Aquarium filled with water (and other living inhabitants)
Step 1: Cut your 2Liter bottle in half or at an angle (like the above image). You can choose to discard the base at this time or keep it to support the top half of the bottle until the unit is complete.
Step 2: Using a thick piece of Wood/Glass/Plastic (your choice as long as it can sit on top your tank), drill a hole that will fit your bottle cap. You'll also want to drill a small hole in the top of your bottle cap (this will allow the roots to be pulled through the base later).
Step 3: Apply the Aquarium Silicon (or adhesive of your choice) inside the drilled hole in your Wood/Glass/Plastic. With the cap screwed tightly onto the top half of your bottle you'll than push it into your drilled hole with the adhesive in the Wood/Glass/Plastic. Be sure to position the top half of the bottle the way you want your planter to be displayed later once the adhesive has dried completely.
Step 4: Once the adhesive is dry you can than choose to unscrew the bottle from your Wood/Glass/Plastic and the cap should stay securely inside the Wood/Glass/Plastic hole. This will help if you ever want to clean out your planter later on.
Step 5: You can then add your plant into the top of the 2Liter bottle, make sure it is screwed securely back in place on your Wood/Glass/Plastic. Be sure the roots have been pulled through your unit and can reach the water in the aquarium.
Note: You don't need the tank lid to be as wide as the tank so long as it can rest safely on top the tank length wise. You could also make a hinged door that can be lifted up to feed your fish if you do decide to cover the entire top of the aquarium. Or even create a box type hood where you could attach lighting underneath for an all around planted or lighted aquarium.