Since I was a little kid ecosystems have fascinated me. We did the ecosystem in a 2-liter bottle project in science class and I was hooked. I filled jars with ants and brought snails home from the beach. My parents bought me a plastic bathtub for my baby dolls and I filled it with moss and slugs and nightcrawlers. Nature fascinates me.
A saltwater fish tank is a working ecosystem. Waste from fish is taken care of by crabs, waste from the crabs is eliminated by snails, and so on, right down to the tiniest bacteria in the tank.
When you begin a saltwater tank you first buy a piece of live rock. The rock itself is not alive, but crawling all over it are many micro and macroscopic life forms. You truly never know what you are going to get in a piece of live rock. The first piece of live rock I put in my tank has a lot of holes and nooks and crannies in it. I chose it on purpose because the tank is so small, this creates more surface area for crabs and fish to establish territories.
However, hidden inside all those nooks and crannies were a lot of different organisms I didn't know I was purchasing. I've had my saltwater tank for two years and still animals come crawling out of that rock that I had no idea were in it. A few months ago my tank began sprouting invasive anemones that were killing the plants and coral I have in there. Just the other night, after an entire day spent cleaning my tanks, a 5 inch long fireworm came crawling out from under the rocks.
Fireworms (or bristleworms) look like caterpillars, can grow to 24 inches long, and reproduce easily. They also have bright red stingers down their sides which will embed in our finger like a shard of glass. Luckily fireworms are nocturnal so they had never given me trouble, however as they grow they can damage coral, plants, and small creatures like my beloved blue-legged hermit crabs.
The beautiful thing about saltwater tanks is that it largely takes care of itself. Got fireworms? Get an arrow crab or a wrasse (a kind of fish) and they will eat the fireworm. Accidently overfed the tank? No worries, a few snails will clean up the mess for you. This is what fascinates me about my tank, particularly my saltwater tank. It is a small self-contained ecosystem. My own beautiful mini-ocean in my living room.
I have three tanks and I love them all. My little bedroom tank was my grandmother's and I keep it up in honor of her. My 75 gallon tank covers half of a living room wall and has two fish in it, both over a foot long. But my saltwater tank is by far the most fascinating tank for me. With constant activity and amazing creatures I anticipate a surprise everytime I look at it.