There are no hard rules how one should use the catappa leaves, it's all depends on individual experience. Here I can only write about my personal experience on using the catappa leaves, and base my conclusions on reports gathered from other fish keepers in Singapore.
What fish can I use with the leaves ?
It has proven that the leaves are suitable for Flower Horn Cichlids, Arowana, Blood Parrot Cichlids, Discus, most tropical fishes, especially River Based fishes, and the leaves is almost a must particularly for Siamese fighting fish (Betta Splendens), also known as the Betta (especially in the US).
In what form of the leaves can I use ?
Besides using a whole piece of leaf itself there are currently quite a few sub-products that are made from the leaves for sale online. Some crushed the dried leaves and seal them into the form of Tea Bags, some boiled the leaves for the so called "Blackwater Extract", and now there is even something called Bio-Leaves, where the leaves are being grind in to the finest of powder form. So, which one is right for you ?
Use the Leaf Directly
The most simple and direct method is to put the leaves into where your fish are, so called the "aquarium". Here the word "aquarium" can be refer to as a jar, a container, or a completely set-up fresh-water tank. But remember, the catappa leaves WILL NOT have any effects in a Marine (salt water) aquarium, so never add any of these leaves into your marine tank as the salt water will completelly eliminate the effects of the tannins released by the leaves. For Siamese Betta fish, or sometimes called Betta fish or fighting fish in South East Asia, most of us keep the fish in a glass jar or plastic container, so we just put a piece of leaf into the jar for a week or two, by the 2nd day or 3rd, the color of the water will change into very tan, just like the Black English Tea that we drink. We will still leave the leaf inside the jar as it provide a hiding panel for the Betta fish, the whole idea is to create the natural environment as close as possible to where the fish was habita in the wild. And if you have shrimp in the tank you will see that they love to hide among or nibble on the leaves along with other fishes.
How much should I use if I am using the whole leaf ?
For a size of a normal glass jar (similar to that of a Peanut Butter Jam glass container), one single leaf of any size will do all the magic for 2 weeks. If it is a container with about 15 litres (or 4 US gallons), two leaves would be ideal. Below is a guide of the quantity of the leaves (in whole piece) to be use :
These recommendations are based on 15-25 cm ( 6-10in ) leaves. If your leaves are larger or smaller, you need to adjust accordingly.
But as a rule of thumb, you may want to use a couple of small leaves or half to a quarter of a large leaf for a 4 gallons betta tank. And if you are spawning the bettas, I would recommend sinking one or more leaves (for the female to hide in), allowing at least one leaf to float. The male betta will probably build its nest under the floating leaf ! With the bubble-nest pushing up the leaf, it will float for long enough for the fries to hatch and become free swimming.
If you are using the leaves for a larger community tank, I would recommend putting 2-3 large (or 4-6 small) leaves for every 100 litres (25 US gallons). You do not really need to change the leaves until they begin to disintegrate in about 3 weeks time. In fact, if you have shrimps or plecos, they will devour them when they start to disintegrate.
For how long ?
Catappa Leaves normally disintegrate after 3 or 4 weeks, depending on how interested your fish is in actively destroying them. Betta keepers that shun torn leaves normally replace the leaf every two weeks, but for aesthetical reasons only. If your don’t have a problem with seeing decaying leaves in your tank you don’t have to change them more frequently than a month. Unless you attach the leaves to something, they will normally float at the surface for 2-3 days before sinking to the bottom. This is a perfectly natural process and you don’t have to replace the leaves just because they sink.
Any side issues when using a whole leaf ?
When the leaves start to disintegrate, or decomposed, it may cause problem to some filtering system, especially if you are using those Corner Filters/Box Filters, or Sponge Filter. The decomposed leaves will quickly block-up your filter, slowing down the filteration and water flow. If it is for a single Betta inside a jar then it is fine. For those bigger tank that use a filter, someone comes up with the idea of the Tea Bags.
Tea Bags with Crushed Catappa Leaves
The adventage of using Tea Bags is you still get the tannins from the leaves (in fact with the leaves crushed it may even be more easy for them to released the tannins) and you will NEVER have any problems with your filter, no matter what kind of filter you are using, because all the crushed leaves are contained inside the tea bags. And when it is time to change you simply get rid of the old bag and replace it with a fresh one. The only set-back I think is that your fish or shrimp will not have any new playground to play with, and your fish tank will not look as natural as it is if the tea bags are to be seen, for me I always keep the tea bags inside my filter, so that at the same time while I enjoy the water condition improved with the tannins from the leaves, it will not affect the look of my fish tank.
But be careful not to use any catappa leaves along with filtering system that contain Activated carbon, because the Burigen will decrease the effect of the leaves, so you must remove any Activated carbon before you add any leaves into your tank.
You can always use the Tea Bags on larger tank but not for a single Betta in a small jar, as the fish will not feel natural and comfortable, use a whole leaf instead. For larger tank with filter you can always insert the tea bags into the filter.
How much bags and for how long ?
It all depends on the size of the tea bags and the size of your tank when you determind how much bags you should use. A simple way of calculating is a single 2.5" x 4" size tea bag for 2 to 5 gallons of water. So if you have a 20 gallions size tank, you should use about 4 or 5 tea bags. Just remember the ratio of 1 packet of 2.5" x 4" size tea bag for 5 gallions of water, and multiply from there.
Since your fish will not be playing with the tea bags your main purpose is for the tannins to improve the water condition and the fish health, it would be ideal to replace any tea bags after used for 2 to 3 weeks.
I have tried the method of boiling the leaves in order to so call extract the tannins by using heat, and for my own experience the result is not that promising. My fishes somehow don't show any happy signs with forced extract, compare to the usual method by leaving the leaves in the water and release the tannins slowly and gradually. They will all hide into a corner and stay low in the tank, perhaps by the sudden change of the water color. The PH in the water also did not shows much different. Compare with putting a whole leaf or using some Tea Bags, the change in PH is much obvious and my fishes are much active. I don't know why is that so but I think it has got to do with Oxidation. With the leaves in a cooking pot and under some hot boiling water, probably most of the natural goodies with-in the leaves will not last too long or retain after being Oxidation. This is only my own guessing, you must try it out and see for yourself.