So, here's the tank reason, and then I'll blow into some ecology for you.
When you get a tank, you start with no populations of anything. You get live rock to form the basis of the biodiversity - and remember that virtually everything is moderated by bacteria and photosynthesis in our tanks. So liverock is the substrate for all this stuff, and also has a lot of life on it. How much depends on a lot of things. Mostly, marine animals and plants don;t like to be out of water for a day at a time...much less the many days to sometimes a week that often happens. So, assuming you are not using existing rock form a tank, or the well-treated aquacultured stuff, you have live rock that has either relatively free of anything alive, or you have live rock with a few stragglers and a whole lot of stuff dying or about to die because it won;t survive in the tank. From the moment you start, you are in the negative. Corallines will be dying, sponges, dead worms and crustaceans and echinoids and bivavles, many of which are in the rock and you won't ever see. Not to mention the algae, cyanobacteria, and bacteria...most of whcih is dead and will decompose, or which will die and decompose. This is where the exisitng 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. 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 decompositon 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 cause there are enough bacteria present to keep upwitht he nitrogen being released by the dying stuff....does not mean things are finished decomposing.
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...prevents some microbes from doing very well. Same with the algae, the inverts the
cyano, the dinoflagellates, etc. So, let's just figure this death and decomposition is gonna 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. Ooops. 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 aleady a huge population...so you never test ammonia..."The water tests fine"
But, all these swings are happening...every time, they get less and less, but they keep happening. Eventually, they slow and stabilize. What's left? A tank with limited denitrification and a whole lot of other stuff in the water. Who comes to the rescue and thrives? The next fastest growing groups...
cyano's, single celled algae, protists, ciliates, etc. Then they do their little cycle thing. And then the turf algae. Turfs get mowed dow by all the little amphipods that are suddenly springing up cause they have a food source. Maybe you've boght some snails by now, too. And 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 succession kick in, and eventually you have a good algal biomass that handles nitrogen, the bacteria have long settled in and also deal with nutrients, and the aquarium keeper has probably stopped adding fish for a spell cause they keep dying and they started to visit boards and read books and get the knack of the tank a bit. They have probably also added abunch 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.
About a year into it, the sand bed is productive and has stratified, water quality is stable, and the aquarist has bought a few more powerheads, understand water quality a bit, corallines and algae, if not corals and other things are photosynthesizing well, and the tank is "mature." That's when fish stop dying when you buy them (at least the cyanide free ones) and corals start to live and grow and I stop getting posts about "I just bought a coral and its dying and my tank is two months old" and they start actually answering some questions here and there.
So, ecologically, this is successional population dynamics. Its normal, and it happens when there is a hurricane or a fire, or whatever. In nature though, you have pioneer speices that are eventually replaced by climax communities. We usually try and stock immediately with climax species. And find it doesn't always work. Now, the "too mature" system is the old tank syndrome. Happens in nature, too. That whole forest fire reinvirograting the system is true. Equally true on coral reefs where the intermediate disturbacne hypothesis is the running thought on why coral reefs maintain very high diversity...theya re stable, but not too stable, and require storms, but not catastrophic ones....predation, but not a giant blanket of crown of thorns, mass bleaching, or loss of key herbivores.
