Often see people saying that their tank crashed and they have started over. Obviously, there are multiple reasons why it could, but I am surprised by how many people have had to deal with this. Is there something in particular that usually causes it to happen?
Usually it is a matter of probability, which basically is - even if something is unlikely to go wrong, it will always eventually happen if there's a chance of it happening. And unfortunately our systems are not exactly well-designed to prevent cascading effects if it does happen, though probably not to generalize too much, this would apply more to smaller than larger tanks.
Take my aquarium. It was running well for six months, and then my anemone got brown jelly disease, and then suddenly everything died. What changed? I added another light about two weeks prior. Highly doubt that was the case, because well, I have changed light settings a lot. But could be anything else. Maybe there was a particularly hot day that I was not aware of, and temperatures maybe rose too high for a few hours. Maybe a chemical I use was off. Maybe it had to do with my salt. Maybe the water was not filtered properly before mixing salt. Maybe some sort of organism finally grew enough in the aquarium to cause some sort of cascading issue. Maybe I was opening the window less and pH dropped too much one day. Maybe my corals depleted the water of nutrients and I was not feeding enough and they all started to stop doing well at once. Maybe a small organism got into the return pump and got completely smashed apart and released something. Maybe the corals decided to start finally warring.
The problem is that our tanks are not static structures, things change.
Unfortunately a lot of us may never know exactly what happened. For example, I have 0% idea if pH or temperature changed significantly for any period of time, given that I was not recording either constantly. There are equipment for that nowadays of course, but not everyone has it.
Could be some sort of trace element instead. Something could be consumed far faster than water changes was replenishing it. ICP tests would help, but again, not everyone does it.
Things like RODI filters or something 'stuffing up' is harder to figure out.
Of course with some detective work, eventually one might be able to determine what went wrong nonetheless, but the problem is that unless one has a lot of back ups and ways to mitigate problems, which is a lot of us, then yeah one day something could just happen and well, that's that.
I mean back to my example - my fish are 100% unfazed, so are my inverts. Yet suddenly all my corals died off at once. No algae bloom until after they died, so what gave? I did not change my routine at all over the past six months, but I can't imagine it being the light either. I figured it may be an issue with the filter for my water, but even then well, unless I was testing my water each time, then I still can't be 100% sure it was because my water that I used for a water change just before all my corals and anemones died that that is the culprit.
