when I said no other fish, I mean no other fish. About the only time you can get away with a slightly overstocked tank is when you have a good sump filter with tons of biological media. reason being is you normally have quite a bit of extra water was all that media in it.
Okay, regarding the cycling, I'm not sure how I can explain it any easier than I already have. Okay, let me try. This may sound a bit of a crude way of explaining it, but it might make sense.If you were to use your toilet for six weeks without cleaning it, what would start happening apart from looking horrible and smelling badly? Of course the answer is you would have some serious bacteria growing in there. That's pretty easy to understand isn't it? Now think of the inside of your filter in the same way. You have lots of media which consists of either sponge, or ceramic tight media, or both. When the water passes through the filtration system it contains ammonia which is produced by the fish going to the toiletand other things like breathing and in its urime. After a while of the water passing through all the new media, bacteria start to build up (just like it would in your toilet) this bacteria feeds on ammonia. So if you have a regular supply of ammonia, i.e. fish in your tank, you will have bacteria living inside your filtration system.
In a brand new filter you obviously have no bacteria because it's never been on a fish tank before. You have to run the filter for up to six weeks before there is enough bacteria to deal with a small amount of fish. Then gradually, more and more bacteria will build up until the filtration system can handle quite a large amount of waste being produced by the fish.
Now, I mention that ammonia is necessaryto create the bacteria. This is a very dangerous time for fish because as they produce ammonia, there is no bacteria to deal with it site stayers in the tank water. This is why I don't recommend using Oscars to cycle a tank. They don't do well in poor water conditions. You are much better off using quite a few smaller fish. They will produce the ammonia you want to kickstart your filtration system. Many of them will probably die in the process and the rest of them may and up with health problems. Some of them may live happy lives if they aren't eaten by the Oscars.
During the six-week period when you are cycling your tank, if you are using fish, you have got to do regular water changes to stop the ammonia getting too high. Even though you need the ammonia, you can't let it get so high that it kills all your fish. This is why it takes so long, if you were to do a fishless cycle using pure ammonia you could let the ammonia get to very very high levels, you can cycle of tank and almost no time at all. The more the ammonia, the faster the bacteria buildup.
Remember that fish produce ammonia, one type of bacteria eats the ammonia and that is processed into nitrite (another very toxic elements)then you have another type of bacteria that eats the nitrite. When they've done that, it is converted into nitrate. Nitrate is much less toxic and is only removed by doing water changes
Okay, I hope this is a little clearer for you, if you still don't understand how the cycle process works than not sure I can help any more.
