As one who is a big fan of incorporating leaf litter in his biotope aquariums, I have learned a few things over time about them. Even though these are natural items, and no two are alike, there are certain predictable "behaviors", if you will, which make incorporating them into your aquaecapes a bit more predictable.
First off, I'm constantly asked how many leaves you need to get a certain degree of "tint" in your aquarium. That's a really tough one, because there are a few variables, such as the composition of yo ur water, the size of the aquarium, water movement, where you incorporate them in your system(i.e., in a filter chamber, canister, reactor, or the display itself), and wether or not you utilize some form of chemical filtration (such as activated carbon, etc.), to name a few.
I can tell you, as a sweeping generality, that it typically takes me at least 5-5 medium-sized Catappa leaves to get a decent level of "tint" in an aquarium of say 30-50 U.S. gallons, when simply placed in the tank. Guava and Loquat leaves tend to impart a less significant tint to the water in my experience- an almost yellowish-gold color, so you can use more of them, especially in conjunction with Catappa, to achieve a great affect!
In my display tank in my office, I use a ratio of roughly 3 Catappa leaves for every 5 Guava leaves, and that gives me a nice color that I like. Other hobbyists will simply go with 100% Catappa, which I've done in the past, achieving a beautiful golden brown color.
This is where it's going to get interesting soon. We're sourcing Catappa and other leaves from a variety of trusted sources, with the eventual goal of being able to "grade" them based on their ability of a single leave to impart tint to a set quantity of RO/DI water. It will hardly be scientific, but it may serve to give our customers and enthusiasts at least some basis for a starting point. The ultimate goal will be to have wide variety of "gourmet" leaves, if you will, from a variety of sources, so you will be able to select based on the appearance of the leaves, where they were sourced, their sizes, and their "tint capability", among other factors. We're already well on the way and will be rolling this out in the near future!
Now, I've already touched on the issue of chemical flirtation and it's impact on removing the tint and tannins associated with leaves. It' is entirely possible to minimize or render the ph-lowering and water-tinting capability of tannins released by leaves with activated carbon, Purigen, or other chemical filtration media. Tannins are rather weak acids, yet they can lower the pH of water when their is less "buffer" in the system (i.e.; lower general hardness). Just how much tannins can lower pH in a given system depends upon how much buffering capacity the water has.
With "harder" water (i.e., water with a greater buffering capacity), you can have the tinted water look from leaves and wood, without the pH reducing effects, particularly if the tannic acids are absorbed by the aforementioned chemical filtration media. So, in other words, you can have the aesthetics of blackwater while running your aquarium at a higher pH if the hardness is sufficient. Once you remove tannins in a lower hardness system, your pH should rise, too, since you're removing the acids.
Woah. Head spin time.
Bottom line is this: You can use chemical filtration media in "tinted" tanks. However, depending upon the amount of media, quantity of tannin-producing items (leaves, wood, botanicals, etc.) and the capacity of the aquarium, the impact will be variable. I run chemical filtration media (Purigen and activated carbon) in my tanks, and I still have nice tint and pH relatively stable at around 6.8.
How often do you need to replace your leaves? Well, another great question for which there is no "rule" involved. The reality is that you can simply add new leaves on a regular basis, so you'll always be making up for the ones that have decomposed. Some hobbyists like to remove the decomposed leaves, preferring a more "pristine" look. It boils down to aesthetics, really.
I like the natural affect provided by leaves in various states of decomposition. It looks and function just like a real tropical stream bottom, providing shelter and food for a variety of animals.
How deep, how dark? It's really a matter of aesthetics.
Explore. Create. Evolve.
And Stay Wet.