First, here's what you need for each one:
3 clear 2-liter (empty) soda bottles
clear packing tape
4" piece of netting (I used cut up pantyhose)
fish, snails, or other aquatic life
elodea, (which is banned in CA), duck weed (which the store didn't have), anachris, or other aquatic plants
crickets, pill bugs, earthworms
a few dead leaves and small sticks
You can use the link to Scribbit for perfectly detailed directions, but first you get your soda bottles, 3 for each ecosystem, and you begin cutting them up. The bottom one (basement, AKA pond life) gets the top cut off of it. The middle one (AKA coupler) gets both bottom AND top cut off. The top level (AKA earth level) gets bottom cut off, but saved to reattach at the end.
Once you have your pieces, you take your basement/pond life level and add an inch or so of gravel. Fill it almost to the top with water and add a few drops of dechlorinator. Then add special friends, like the plants, guppies, and snails. You can use different fish, but guppies are the sturdiest. These snails that the nice fish store man chose for us stay fairly small, and they're beautiful. Each ecosystem got 2 snails, 2 branches of anachris (plant), which they could break into smaller pieces, and 3 or 4 guppies so that hopefully 2 will live.
The guppies and snails were happy almost immediately, exploring their new home. Well, except for one of Aiden's that just floated to the bottom. And then swam like a madfish to the top only to float to the bottom again. Aiden named him "Wheeeeeee!" We're not sure how long Wheeeee! will be around for.
For the top/earth level, we put the layer of pantyhose over the open mouth of the bottle, secured it with a rubberband and then turned it upside down. Next, we added a layer of gravel, and then some super-soil from our garden. Into this level we planted some grass seed, but rye/alfalfa/mustard would work well too. I just couldn't find my sprouting seeds. Then we added our friends, one cricket (with a small chunk of potato for him to nibble), 3 pill bugs, and an earthworm each. Then a few dead leaves and small sticks went on top. Just for fun, I gave each child a garden tag to use for a nameplate.
The bottom of that soda bottle (which is now the top) was taped back on, with the edges tucked just inside the rest of the bottle so water doesn't run out. Then that top level is set inside the coupler level and taped.
Both of those two top levels are now set inside the pond life basement level and secured with tape. Now it should be airtight and self-contained, as a proper ecosystem should be.
Now just set them in front of a sunny window so that photosynthesis can happen, setting in motion the cycle of life. The water will evaporate to the top to rain down on the soil, helping the grass to grow and the dead leaves to decompose, which feed the insects. There will be plenty of oxygen from the plants, and the algae that will grow will feed the fish and snails.
Can't wait to watch it all happen!