To expound on the flutuations here is an explanation from Eric Borneman on tank maturity.
"From the moment you start, you are in the negative. Corallines will be dying, sponges, dead worms and crustaceans and echinoids and bivalves, many of which are in the rock and you won't ever see. Not to mention the algae, cyanobacteria, and bacteria, most of which is dehydrated, dead or dying, and will decompose. This is where the existing bacteria get kick started. Bacteria grow really fast, and so they are able to grow to levels that are capable of uptaking nitrogen within...well, the cycling time of a few weeks to a month or so. The “starter bacteria” products give me a chuckle. Anyone with a passing knowledge of microbiology would realize that for a product to contain live bacteria in a medium that sustains it would quickly turn into a nearly solid mass of bacteria, and if the medium is such that it keeps them inactive, then the amount of bacteria in a bottle is like adding a grain of salt to the ocean compared to what is going to happen quickly in a tank with live rock in it.
However, if you realize the doubling time of these bugs, you would know that in a month, you should have a tank packed full of bacteria and no room for water. That means something is killing or eating bacteria. Also realize that if you have a tank with constant decomposition happening at a rate high enough to spike ammonia off the scale, you have a lot of bacteria food...way more than you will when things stop dying off and decomposing. So, bacterial growth may have caught up with the level of nitrogen being produced, but things are still dying...you just test zero for ammonia because there are enough bacteria present to keep up with the nitrogen being released by the dying stuff. It does not necessarily mean things are finished decomposing or that ammonia is not being produced.
Now, if things are decomposing, they are releasing more than ammonia. Guess what dead sponges release? All their toxic metabolites. Guess what else? All their natural antibiotic compounds which prevents some microbes from doing very well. Same with the algae, the inverts, the cyano, the dinoflagellates, etc. They all produce things that can be toxic – and sometimes toxic to things we want, and sometimes to things we don’t want. So, let's just figure this death and decomposition is going take a while.
OK, so now we have a tank packed with some kinds of bacteria, probably not much of others. Eventually the death stops. Now, what happens to all that biomass of bacteria without a food source? They die. Some continue on at an equilibrium level with the amount of nutrients available. And, denitrification is a slow process. Guess what else? Bacteria also have antibiotics, toxins, etc. all released when they die. But, the die-off is slow, relative to the loss of nutrients, and there is already a huge population, and yet you never test ammonia. "The water tests fine.” But, all these swings are happening. Swings of death, followed by growth until limited, then death again, then nutrients available for growth, and then limitation and death. But, every time, they get less and less, but they keep happening – even in mature tanks. Eventually, they slow and stabilize.
What's left? A tank with limited denitrification (because its slow and aerobic things happen fast) and a whole lot of other stuff in the water. Who comes to the rescue and thrives during these cycles? The next fastest growing groups...cyanobacteria, single celled algae, protists, ciliates, etc. Then they do their little cycle thing. And then the turf algae take advantage of the nutrients (the hair algae stage). Turfs get mowed down by all the little amphipods that are suddenly springing up because they have a food source. Maybe you've bought some snails by now, too, or a fish. And the fish dies, of course, because it may not have ammonia to contend with, but is has water filled with things we can't and don't test for...plus, beginning aquarists usually skimp on lights and pumps initially, and haven't figured out that alkalinity test, so pH and O2 are probably swinging wildly at this point.
So, the algae successions kick in, and eventually you have a good algal biomass that handles nitrogen, produces oxygen through photosynthesis, takes up the metabolic CO2 of all the other heterotrophs you can’t see, the bacteria have long settled in and also deal with nutrients, and the aquarium keeper has probably stopped adding fish for a spell because they keep dying. Maybe they started to visit boards and read books and get the knack of the tank a bit. They have probably also added a bunch of fix-it-quick chemicals that didn’t help any, either. Also, they are probably scared to add corals that would actually help with the photosynthesis and nutrient uptake, or they have packed in corals that aren't tolerant of those conditions.
