Originally Posted by Fishyfins

yeah, were told to say that. its a load of tripe. a tank will typically take 4-6 weeks to be ready for fish. here is the explaination. its quite long, so please forgive me XD

an aquarium filter works much like a human sewage works. various types of good bacteria live inside the filter, and devour the waste the fish produce, breaking it down into harmless compounds.
when you first set up an aquarium, there is no bacteria there. as a result, if you put a fish into the tank, its waste isnt eaten and broken down, so waste levels rise. its very common for waste levels to rise to such an extent over a matter of days, that the fish is either killed outright through toxic shock, or its metabolism is severely damaged, and it dies a slow painful death over perhaps several years.
now, you have probably been given something like Stess Zyme, or equivelent bacterial starter, so you may now be thinking "why cant i just use that". well, you can. but the problem is, the bacteria needs food to survive. if you put that in right now, then there is nothing producing the food it needs, so it dies off within a day or two. you add fish after a week, and your bacteria is mostly dead, so your back to square one. another option is to add the fish to produce the ammonia to feed the bacteria, but again, this isnt such a good idea, because the bacteria still needs time to grow, and the fish will be swimming in their own filth for a while, causing them harm.

the only true way of cycling a tank harmlessly is what we call the fishless cycle. in this method, you add small amounts of the bacteria daily to the aquarium, as well as adding small amounts of food for it, usually in the form of neat ammonia bought from a hardware shop. so, you have ammonia in there feeding the bacteria, but the fish arnt being harmed in the process. its a slow process, as it takes 4 to 6 weeks to finish before you can add fish to the tank, but it means less fish get hurt, and the tank turns out a lot more stable in the end.

so yeah, adding fish after 7 days is a recipe for disaster, even if you add the chemicals they tell you to add. even if the fish arnt killed, and you think you have gotten away with it, then the fish usually suffer internal damage which takes its time to kill the fish. this method is not good for the fish.