pH Drops: A Sudden Deadly Catastrophe
If you experience fish jumping excessively or scratching themselves against the gravel or aquarium ornaments, it is quite likely that you are experiencing a pH drop. It can come on suddenly with nothing to trigger it, and can often be deadly, if not by killing the fish directly, then by weakening them and allowing secondary infections to take hold. Here's how:
In an established aquarium, the pH tends to drop
rapidly from fish wastes. The only thing that fights this is the kH buffer. It
acts much like a sponge, absorbing all the acid and keeping the pH neutral. So,
the aquarium could be producing a whole lot of acid, but it's all being
absorbed, and the aquarium is safe.
As soon as the kH "sponge" is full, it starts to overflow all of the
sudden, and the pH will drop. If you have low kH (soft) water, the
"sponge" will be very small and fill up quickly. Adding a shell
or some sort of crushed coral to the tank will leech more of this kH
"sponge" into the water, which will give you more acid aborption
capacity. Also, changing water will replace part of the "sponge" with
a new, empty one. I'd change quite a bit of the water, like 30%, and keep
changing it every 2 days or so, as well as adding the shells or coral.
If you see fish flashing or jumping, you should immediately test pH, especially if you have soft water. If you have hard water, you can still experience this. kH is not necessarily resultant of hard water, but it is usually so. Also remember that pH can be far from neutral without harming the fish. Some fish do quite well in abnormal pHs, but it is a change in pH that hurts them. If the pH seems to not have changed and is near the fish's pH tolerance (you can find most fish's pH preferences here) it's likely that you have some sort of parasite, you can try looking at my Disease Index for more specific symptoms.