Ketapang or Sea Almond or Indian Almond or Terminalia catappa leaves are known to most, if not all Asian breeders of blackwater tropical fishes, to be one of the best water conditioners to promote healing and breeding. They are known to have anti-bacteria and anti-fungal properties. Bettas are known to be induced to spawn by just putting a few of the leaves into their tank. Fish suffering finrot or injuries (such as spawning injuries) will definitely benefit from having the leaves in their quarantine tank. But they can be used for the usual aquariums too.
When soaked in water these leaves will leach a strong brown dye that is full of organic acids like humic acids and tannic acids. These may be useful for inhibiting many types of bacteria as well as to detoxify harmful heavy metals found in the aquarium.
It is no wonder that these leaves are often called "Miracle Leaves"
There are at least 5 ways of using them in the aquarium (apart from using them as beautiful leaf-litter in terrariums for frogs, hermit crabs, scorpions and snakes).
This method is simple: Simply put the leaves into your aquarium. After 1-3 days, if your leaves are completely soaked, they will be water-logged and sink. Apart from their benefical effects on the water, they will tan the water slightly (to a clear amber) and provide a very natural stream-bottom look to your aquarium.
How many leaves to use? Well, the simple answer is: As many as you like. But the more leaves you put the darker will be the water and the lower the PH will be (though it is unlikely to drop below 6.0). You can even use the leaves as the substrate in the tank together with bog wood or a tree stump with roots to create a fantastic Amazon look for blackwater fishes.
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 1 gallon (4 litre) betta tank. If you are spawning the bettas, I would recommend sinking one or more leaves (for the female to hide in) and 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 tank (say, for tetras, gouramis, arrowanas, apistos, plecos, or shrimps), I would recommend putting 2-3 large (or 4-6 small) leaves for every 25 gallons (100 litres).
You do not really need to change the leaves until they begin to disintegrate in about 3 weeks to a month. In fact, if you have shrimps or plecos, they will devour them when they start to disintegrate.
In this method, you will need to get a filter bag (laundry netting or ladies stockings will also do). Crumple up 2-3 large leaves for every 25 gallons, and stuff it into the filter bag. Leave the bag in a compartment of the Overhead Filter near the inlet into the filter. If you use a Canister Filter, leave it in a bottom tray. (Do not simply stuff the leaves into the canister at the top, or you may end up with damaged impellers). The leaves will begin to tan the water in a couple of days. But it will disintegrate over 14-21 days and should be changed.
In this method, you will need a bucket or tub of water. Soak the leaves in the bucket. After a few days, you can pour the very tan water into your tank, and then top up (the bucket) with fresh water.
You can also add a spoon full of salt into the bucket (as preservative), and also run airpump to provide circulation and aeration for better results.
Using this method, you can leave the leaves in the bucket until the water from it ceases to be amber coloured.
This method is described in a separate guide: How to brew your own catappa blackwater extract
In this method, you will make Indian Almond Tea using either Indian Almond Teabags or simply using the leaves directly.
If you use Teabags, make sure that the bags are hygenically produced with water insoluble material. You may buy such teabags on Ebay. Most teabags contain only one to one and a half leaf.
Put the teabag or a large leaf crumpled up into a cup, and add hot water. Leave it to cool. The water in the cup will have the appearance of strong tea by the time the water cools. Add what you need into the tank, and keep the rest (together with the leaf or the teabag) in the refrigerator.
Which method is better? It is really up to you!
Remember to wash the leaves thoroughly if they are not already washed by your supplier. If they are washed, just rinse what you need over running water before using them.
Remember that quality leaves that have not been weather-beaten tend to take longer to leach out their beneficial properties, so be patient. If your leaves are weather-beaten, they may leach tannins very quickly, but most of the beneficial properties would already have been washed off by the rain before they are collected.