To use our fish tank in this manner, we need only have live fish in it. Fish possess an organ called a swim bladder. It's a large sack of air in the fish's innards that helps to keep it neutrally buoyant. It basically helps the fish to remain where it is in the water, without sinking or floating. The size and shape of this bladder can vary depending on the species of fish. Many catfish have a small one, or none at all, to keep them on or near the bottom of the water they are living in. Others, like the flying fish, have a very large one that can help keep them near the surface of the water at all times.
So what does this have to do with the weather? Because fish rely so heavily on this organ, just to remain in one place, changes in air pressure can have gigantic effects on a fish. If the air pressure goes up, a fish's bladder becomes slightly compressed. I'm sure the fish feels like it has really bad gas. The result is a sluggish fish that doesn't move too much. The fish would be "weighted down" and would not be able to perform at it's peak. On the flip side of this, when the air pressure drops, a fish has the opposite reaction. Many fish can perform better and this shows in their activity. As any good fishing guide could tell you fish become much more active when the air pressure drops. If your normally peaceful and calm aquarium, suddenly becomes an active parade of color, you can bet the air pressure dropped and there is some weather on the way.
One fish in the pet industry displays this attribute so readily, it's been called the weather loach. A slender eel like catfish, that prefers to hide almost all the time. It burrows in the gravel or under decorations, unless the air pressure has dropped. When weather is on the way the weather loach, also called a dojo loach, becomes extremely active. This is a fish you never see in your tank, that suddenly is racing and darting the whole length of the aquarium.
While owning an aquarium might not be the ideal barometer for weather predicting purposes, it is a fun trick if you already have one in your home. The next time you know the weather is coming, check out your tank, you might be surprised at what you find. You can see for yourself that when air pressure drops, your fish become happy!