There Was an Old Lady Who Swallowed a Tank

There's an old song about an old lady swallowing a fly. We don't know why she swallowed the fly. Regardless, to catch the fly, she swallowed a spider. To catch the spider, she swallowed a bird, then a cat, then a dog, and on and on until she attempted to swallow a horse. She's dead, of course. 

While it's meant to be nonsensical, it gives an interesting scope into how aquariums fail. 

A stable and flourishing ecosystem rarely collapses on its own. The biological engine of life doesn't turn on and off, it just keeps running. It must keep humming along without stop. When left undisturbed, an ecosystem runs in balance. 

The thing that crashes tanks is usually the tank's caretaker. Humans have a really hard time leaving things alone. It's especially stark maintaining an old and mature tank versus a brand-new and fragile tank. New tanks are heavily reliant on regular maintenance to not become toxic to aquatic life. Mature tanks quietly tick along without much intervention. 

Here's the problem that most people encounter. The fly is usually a small and seemingly insignificant decision. You don't notice a real problem until the horse kills it. 

I watched a video where a YouTuber takes you through his own beach paludarium crashing. He details how a few bad decisions compounded into a total collapse. It all starts with adding a fish he probably shouldn't have added. While it wasn't the fish itself that killed everything, this particular species needs to be fed constantly. Feeding was ramped up to accommodate. This caused extra nutrients to fuel algae. The algae was at risk for nuking the tank, so he added an urchin to eat it. The urchin worked too well and ate all the algae. Pest anemones and algae took over. He added a fish to take care of the pest anemones, which ate the remaining corals, too. The full collapse happened during a weekend out of town where his heater went down. 

Had he not kept trying to change things, the heater failure would have been buffered out. The ecosystem existing on a knifes edge can't take as many flies as the one that has layers of complexity baked in. Since, after all, we all swallow flies from time to time.