How to build an airborne ecosystem
I will admit to plagiarising Josh King's airborne ecosystem. However, in my defense, the sugars have to either come from air or from space, so any answer has to incorporate those factors. I will explore how a plant could be made to have an airborne ecosystem, but not one of the ground.
To not have plants, you can't have sunlight
To not have sunlight, you must either a. be very, very far from a star or b. you must have heavy cloud cover. Turns out, you can do both together.
Venus has a series of high clouds made of sulfuric acid. These clouds cover the planet completely and are opaque to most forms of radiation. Despite being closer to the sun, the optimal light energy received on the surface of Venus is around 10,000. Meanwhile, direct sunlight on Earth is about 100,000 lux. Spacecraft that have landed on Venus are unable to make use of solar panels for energy, due to the low light conditions.
In order to create sulfuric acid, sulfur dioxide (from volcanos), oxygen and water are required. Since you wish to have a 20% oxygen atmosphere, this could work out well. There is no reason to have a majority carbon dioxide atmosphere as Venus does, a nitrogen/oxygen atmosphere with sulfuric acid clouds will work well.
However, there is a requirement to have a high atmospheric density for the sulfuric acid to float. Sulfuric acid haze exists above the 10 atm layer in Venus' atmosphere. If the pressure were lower than this, the sulfuric acid would collect on the ground from rain. Instead, sulfuric acid rain evaporates in the atmosphere at around 25km altitude and is recirculated. In order to have max cloud cover, the atmosphere must be well over 10 atm at ground level to ensure that minimal sulfuric acid rain pools on the ground.
Thick atmosphere can mean greenhouse effect
Depending on your atmospheric composition. If your atmosphere is primarily nitrogen/oxygen, the greenhouse effect will not be that significant. If it is carbon dioxide/oxygen it will be very significant. This will only affect the distance from the star of your planet.
With a nitrogen/oxygen atmosphere, your planet will need to be closer to the sun than the Earth. Due to the high albedo of the sulfur dioxide clouds, most solar energy will be reflected back into space. Venus, for example, receives less solar energy than the Earth, despite being closer, since so much is reflected.
Cloudtop organisms will need to process sulfur compounds
Sulfur-reducing bacteria exist on Earth; they convert sulfate ion (SO$_4^{2-}$) to hydrogen sulfide (H$_2$S). Sulfate ion will be available from sulfuric acid in solution in water droplets. Anywhere water droplets can float, these bacteria could find their food.
Here, an oxygen atmosphere is an advantage since it will recycle the hydrogen sulfide to water and sulfur dioxide, and the sulfur dioxide will further react with molecular oxygen and water to produce sulfuric acid.
Oxygen requires photosynthesis
In order to maintain this oxygen atmosphere, you will need a metabolic process creating oxygen. The only one I can think of that will work is photosynthesis. This is the weakest link in this ecology, in my mind. The photosynthesizers will need to float above the sulfuric acid clouds in order to get sufficient sunlight.
An alternative would be a photosynthesizer that uses long wave infrared light. This light could penetrate the clouds and mean that there would be some low-level photosynthesis happening on the planet's surface (a second food source, beyond the sugar rain). This needn't be too much biological mass, but it has to have been happening long enough to give the planet (with an active geological cycle to keep sulfur in the atmosphere) a 20% oxygen atmosphere.
Conclusions
You will need:
- Sulfuric acid clouds to block sunlight
- High-density atmosphere to keep sulfuric acid off the surface. As a bonus, this makes larger organisms able to float in the clouds (think salps)
- To get a 20% oxygen atmosphere, you need either a nitrogen or carbon dioxide base. A nitrogen planet would be nearer the sun, CO$_2$ farther away to account for the greenhouse effect.
- You need at least trace amounts of water to allow sulfuric acid formation
- You need a sulfur source to keep sulfuric acid in the atmosphere. Volcanos are the usual culprit.
- You need a photosynthesizer, possibly floating or at ground level using long wave infrared light to produce oxygen.
Of course, with all this, there isn't really sugar raining down from the sky; its more like dead bacteria. So you won't end up with a Candyland sugar-crust, but you would end up with plenty of fungus food on the surface, and I'd imagine the fungus could be the base of a complex animal food chain.
You could add some handwavium reason that either a. the sky bacteria just drop sugar for funsies or b. some sky amoeba-like predator eats bacteria and poops down sugar. But given how valuable sugar is as an energy source, neither of those explanations seems reasonable to me.