Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
No. Plants cannot take in giant molecules like fallen fish food or a dead fish, or even the large organic matter that is partially decomposed stuff.
Malaysian Trumpet Snails will eat that organic matter. They do not eat the single atoms or small molecules that plants need.
When the fish food, or fallen leaf, or dead fish goes through their digestive tract it is turned into the small molecules that plants can use and we call fertilizer.
Malaysian Trumpet Snails PRODUCE fertilizer, they do not use it.
Microorganisms in the tank do the same thing. They grow on the fallen food or dead fish and break it down into smaller and smaller pieces. Eventually into the small molecules that plants use.
Same thing happens in a compost pile, or in the garden soil where there are dead leaves, roots and so on, dead worms and bugs, animal waste... The plants cannot use any of it directly. Microorganisms in the soil break it down and end up creating the materials that plants can use. Sometimes the material will pass through more than one critter. An earthworm might eat a bit of fallen fruit, then poop. Microorganisms digest the worm poop. The waste from the microorganisms is fine enough for plants to use.
Each time the material passes from one organism to another some energy is used by that organism, and some of the molecules get used to grow or make more organisms, so what starts off as uh... say 100 grams of dead leaf might become 10 grams of earthworm, then 1 gram of microorganism, then .1 gram of fertilizer for the plants.
The energy part is pretty much gone, plants need the molecules as material to turn them into more plant material. They get their energy from the sun.
Same sort of thing happens in the aquariums.
The soil that you buy or make, and has fertilizers (the small molecules plants use) can also have larger bits of organic matter like sticks, rotting leaves, sawdust, peat mos and manures. None of these are used directly, but all can be broken down into the molecules plants use.
You add more organic matter to keep the cycle going when you feed the fish.
You add more fertilizers when you dose the tank, or put root tablets in the substrate.
These additions replace what the microorganisms, snails, worms and so on have created, that the plants have used.
You remove these materials from the tank when you prune the plants, or do water changes.