Here is a quick message from Dr Tim that helps explain Bacteria.
In your aquarium -
Your fish eats, it then excretes, the waste matter breaks down and produces ammonia (the first part of the nitrogen cycle).
If nothing else came into play for these next steps then your fish will die or be severely distressed by rising levels of ammonia.
So let us introduce you to the stars of the show, the nice guy nitrifying bacteria from the Nitrospira family!
During the cycle process the population of bacteria grows and as they feed on ammonia they reproduce and allow more ammonia to be consumed and furthermore be converted into nitrite and then nitrate (pretty much the end of the nitrogen cycle).
The ball stops here with the capabilities of the nitrifying bacteria and the next part either starts with you doing water changes.
Or you employing the help of other bacterial strains to remove the nitrate by means of providing a anaerobic (oxygen depleted) zone for the different bacteria to live and convert the nitrate into harmless nitrogen gas bubbles that diffuse into the atmosphere.
Or providing a food source to keep more types of nitrate eating bacteria thriving, this is known as organic carbon dosing or "vodka dosing".
This is more advanced and shouldn't be attempted without reading more into the process.
Sub-note: a new aquarium that cycles "fast" from using dry base rock for the bulk of the structure and seeding it with a small amount of live rock or established marinepure isn't usually cycled at all!
There may not have been a significant spike in ammonia, nitrite or nitrate and that often leads to people adding fish too soon.
What's happening in most cases is that the bacterial population hasn't grown, the large water volume in comparison to the amount of live nitrifying bacteria masks the signs of a cycle and needs longer to populate the dry rock structure and sand bed with beneficial bacteria.
Please don't rush your cycle times! One of the reef aquarium mantras that will always ring true is that nothing good happens fast.
Use your cycle time of up to 8 weeks to plan what fish you would like to add and check their compatibility with other. Also now is a good time to get familiar with using water test kits.
Information provided by:
MARINE AQUARIUM FANATICS AUSTRALIA