Angelfish nipping Angelfish

Question
I have an aquarium, with Tetra(8), Barb(1), swordtail(1), Angefish(2).
One of the angelfish has started nipping the fins of the other angelfish (not troubling any other fish).
I have had the angels for about one year, in which they have grown from about 25mm to about 35mm & 40mm length; the small of the two does the nipping.
The tank is 30"x18"x15".
WHat is causing this recent behaviour, and what can i do to stop it?

Answer
Hi Andy,
Thank you for your letter. Here is my page for holistic aquarium care:
http://steamboats.com/aquarium.html
I encourage you to read & print it out for general reference.
If the fish have grown that much, they are probably angry at each other for taking up too much space. Not that fish "think" in the same way we do, but if you were in a little box with another big creature like you and you both kept getting bigger and bigger, you would start to fight back too.
The solution is to move into a larger tank. Hopefully that will cure it. The sooner the better because they might cross some boundaries and get a feud going. If they learn to live in peace in the new tank, remember that fins grow back.
I don't really know what goes on in their little heads, but moving into a bigger tank is the logical answer to the problem.
Now you know why people end up with huge tanks. They have to do it to accommodate growing fish. Ask the people at the aquarium supply shop - they will tell you that their best customers always have to upgrade. It's a difficult hobby, so if your fish are healthy and growing, you are doing very well.
Please consider using the holistic principles (link above) when setting up your new tank, if you decide to do it.
You can probably use the rest of the hardware you already own. Luckily the tank is the least expensive part of the hobby. You may also need more gravel, a bottle of "Cycle," and lots of bottled water.
Write back if you want to discuss further.
Nori