I am one who enjoys tinkering with things, just recently I had to make an underwater dry spot (cover picture) for my fiddler crabs at the bottom of my 55 gallon fish tank, as I said, being that I have to tinker, I got to thinking of how i could make the dry spot into an underwater greenhouse. I have seen a few "natural" filters, basically a bunch of hydroponic plants, some nylons, PVC pipes, and a pond pump. The idea is that you don't need a charcoal filter to clean the nitrates and nitrites from the water, the plants do that for you. But that system is complex and boring. I am shooting for an completely submerged greenhouse to do the same thing, and hopefully I won't have to buy charcoal filters left and right, thus cutting down on the plastic waste from the filter, and a nifty way to reuse jars. :D
Update: I have a plant beginning to grow in the underwater greenhouse :D

Quick list of things to gather:

Fish tank tubing
Glass jar with screw on lid
Anti-siphon thing
Regulator valve
Drill with small bit
Fish tank gravel
Soil
Seeds

for my underwater dry zone visit here:
https://www.instructables.com/id/Under-water-Dry-zone-for-fiddler-crabs/

Step 1: Get a Jar, Preferably Class

Get yourself a glass jar, I personally had a grape jelly jar with a metal jar kicking around. After enjoying many many pieces of toast covered in grape jelly, and the occasional peanut butter and jelly sandwich, I finally had an empty-ish jar. Just wash out the jar and the cover, let dry because the next step will be easier dry.

Step 2: Drilling Time

Ok, take the drill and bit and drill a bunch of small holes in the jar, making sure that they are indeed small (you can pop a nail through the lid and it will perfect size for allowing water in). Make two holes big enough to fit the fish tank air tubing to fit through, preferably on the edge of the jar. Feed the hose through the two holes in the jar. Have one of the hoses reaches the top (bottom) of the jar, the second hose can sit a couple of inches from the lid.

Step 3: Setting Up the Jar

Since the jar is going to be inverted, this is going to be interesting to set up. Take a light little bit of soil, enough to cover the bottom of the jar about 1/4'' deep. Next place your seeds on top of this layer of soil. Followed by the next layer of soil, about a 1/2" should do. Next pour in the aquarium rocks until the overall jar is about 1/4 full.

Step 4: Capping the Jar

Now that you have the jar all set up, we need to cap it. With the jar capped and the end of the hoses hanging out of the cap, we need to connect up the hardware. On the hose that feeds to the bottom (going to be top) of the jar, connect a regulator valve that you can completely close so that no air can escape, on the hose that is barely in the jar, attach the anti-siphon valve with the flow of air traveling into the jar.

Step 5: Place Jar Into Tank of Water

This is the fun part. Right now i have my jar connected to an average aquarium pump, this allows me to adjust how high in the tank the jar floats, keep the regulator valve closed tight, you don't want to flood your jar with water. Slowly open the regulator valve until the water is covering the aquarium rocks just under the soil, then close the regulator valve tightly. The water in the fish tank should provide all the nutrients your plant will need. As for the needed CO2, you can make a CO2 factory using yeast, sugar, and baking soda, with a check valve on the out flow of the jar, you could in theory adjust the level of the water to where you want it, connect up the CO2 factory, and leave it to work.

I will update this with pictures of the plants that grow inside it, and any tweeks that might be needed to be done.

I am open to any helpful comments that you might have :D