As aquatic botanicals set in our systems, we like to say that they recruit "biofilm" on their exposed surfaces over time...And as the softer botanicals, like leaves, break down and decompose, forming a matrix of organic matter, they play host to a diverse assemblage of life forms.
Well, what exactly is this stuff? Is it bad, good, or of no consequence to our fishes? Well, I'll state right off the bat that it's not bad...really. Biofilms are composed of populations or communities of microorganisms adhering to different types of environmental surfaces. They're generally bacteria bound up in a sugar-laced "package" called glycocalyx, which builds up this film. If you think you've seen this stuff before, you're probably right. In fact, biofilms are the same structures that build up on bioballs, filter pads, and other "mechanical" filter media.
In fact, what we call "aufwuchs" in the African cichlid "context", is a collection of simple sugars, bacteria, fine detritus, and algae- all of which comprise a nutritious "package" of food for larval and adult fishes alike...So it forms a rather complex little "micro community" of food sources for grazing fishes.
When your botanicals start to recruit this film, they're doing exactly what they do in nature- enriching the environment. As softer botanicals like leaves break down in the aquarium, protozoans, rotifers, small worms and crustaceans begin to appear in the matrix of decomposing materials.
Could you ask for a better, more natural place to rear fry? I'm not so sure!
I think you get the picture- the appearance of these biofilms in our aquariums is not only a natural process- it's very beneficial to our fishes as a supplementary "feeding substrate." Indeed, the materials found in biofilms may be one of the most important sources of nutrition for our fishes.
So, yes, it may not be the most aesthetically pleasing thing to see leaves decomposing in our tanks to some people- but it's actually a beautiful thing to our fishes.
Yes, depending upon the aesthetic you're trying to achieve, you might want to remove some or all of the decomposing leaves at some point. However, if you're looking for a way to supplement and enrich your young fishes' diet, maintaining some of this decomposing litter in a natural setup is of significant benefit!
So, the choice is yours- depending upon the effect you're trying to achieve in your aquarium, you can either leave it in, or siphon it out of your tank as you see fit.
To put it simply, one aquarist's "rotting leaves" are another's "feeding station!"