Cycling your tank', 'Filter Bacteria', 'New Tank Syndrome' are probably words that you have never even heard of, unless you have a group of friends who have been keeping tanks or have been buying stuff from a good LFS (Local Fish Shop) or you are a serious hobbyist and Internet savvy.
Let us start with 'New Tank Syndrome'
Most fishkeepers in India will be familiar with how the experience starts. We go to the nearest LFS, grab a tank, usually the smallest one we can (as we have a notion that we can't maintain larger tanks), buy an airpump (usually referred to as 'motor') with an airstone (thinking it provides oxygen, which is not perfectly true). Then we also buy a few fish: always goldfish, atleast 4 numbers and if the LFS guy has put the Vaastu (Feng Shui) effect on you, 8 goldfish and 1 black goldfish (black moor), some angels (Oh! they look so cute), and last but not the least, the tank cleaner (pleco) for getting rid of the waste in the tank. If he has a white tank cleaner (albino pleco) one of that too! Don't forget the decor. Yes, those colourful pebbles, the scuba diver or the kid taking a leak, some plastic plants.
Smug with the thought of providing a good home for the fish, compared to the dirty tanks at the LFS, you drive home. Add water straight from the tap or borewell, drop the fish in and give them some feed, which the ever greedy fish seem to gobble up in a few seconds. So you think they are really hungry and you feed more. All seems well for the first two days, and then, disaster strikes. You see one dead fish in your tank and you are at a loss to reason why. The next morning, there are two more dead and that beautiful goldfish (your favourite) doesn't seem to be doing too good. You run to the LFS who gives you a colourful liquid and asks you to add it to the tank. Since you are already at the LFS, you decide to take two more goldfish to replace the ones you lost.
Back home, the tank is smelling bad, so you move all the fishes to a bucket, remove the decor, give them a nice scrub, wipe the tank clean, add more water, in goes the decor and the fish along with the new ones you picked up. The tank looks better than earlier. Maybe it was bad luck, or if you are a firm believer of 'Vaastu', the fish died to save you from some agony. So, that's lucky actually because you are safe.
Two or three days later, you see another dead fish, this time, it is your favourite. You go back to the LFS and argue with him that he sold you bad fish. He will turn the other way and calmly tell you that you selected the fish and it was very much alive all these days. Which means something must have gone wrong at your place for which, he has got nothing to do. You swear never to go back to him again.
You visit another LFS, get some more 'colour' liquid of a different kind, make another water change and add the liquid. You also had picked up fishes from the new LFS, remember, so they go into the tank too. And yet again disaster strikes in a couple of days. You are tired of changing water, removing the dead fish everyday, until the last one is dead too. You are scolded by your parents/spouse for wasting your time and the money. The aquarium is removed and moved to the attic. There ends your tryst with keeping aquariums.
So, What went wrong?
In a single line, you just experienced the 'New Tank Syndrome'. When there are fish in your aquarium, their constant metabolism gives rise to a lot of toxins in the form of ammonia. Bacteria in the convert (digest) this Ammonia into Nitrite. This Nitrite is even more harmful than Ammonia in hardwater. However, another form of bacteria is already at work converting this Nitrite into less toxic Nitrate. But constant accumulation of Nitrate will also cause loss of appetite in fish and stress them, making them vulnerable to disease and parasites. This can be controlled by periodic, partial water changes.
When a new tank is set up, the level of ammonia in the tank tends to be very high with not much bacteria around to remove toxins from fish waste. Bacteria needs time to grow and take control over the toxins in the tank. The process of conversion of Ammonia to Nitrite and then Nitrate is called the 'Nitrogen cycle' and this process needs to be first established, that is, the tanks needs to be 'cycled' before adding fish to the tank.