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I have 8 white skirt tetras, 8 albino cory cats, and had 8 danios. Now I have 7 danios. One mysteriously disappeared. I am very doubtful it is hiding because my danios don't do that; all they do is swim at the surface.

It is impossible for the fish to have eaten the danio. They are too small.

It is impossible for the fish to have jumped out. The tank is fully covered. I checked around the tank, searching for a good 30 minutes - nothing found.

My fish isn't dead. The cory cats would have easily started feasting on it. They eat anything that's at the bottom, essentially. (Perhaps it died while I wasn't around?)

My filter doesn't contain the fish.

No sign of a fish skeleton ANYWHERE.

What could have happened to it?

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    Do you own a cat or dog? Any other pets, or children, visiting toddlers that may have grabbed a fish off the ground and not told anyone?
    – user6796
    Dec 3, 2013 at 1:29
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    It is funny, because the only one with access to the room containing the aquarium is me. No other living thing except me and bacteria.
    – Don Larynx
    Dec 3, 2013 at 1:31
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    Perhaps you have a giant sea worm?
    – JoshDM
    Dec 3, 2013 at 3:36
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    How heavily planted/decorated is your tank? If one died, the current may have gotten it stuck behind or under a plant, log, whatever. Sometimes you just have to start moving things around. I seriously doubt the other fish you mentioned was able to catch a danio, let alone eat it. Also, is your filter intake big enough to swallow a fish?
    – GrandmasterB
    Dec 3, 2013 at 5:02
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    Someone in my family had a similarly impossible disappearance; although his tank was semi-open, he also ruled out most imaginable scenarios. He eventually found that the fish had jumped straight upwards and somehow got instantly burned onto the aquarium's lighting (which, this being 30 years ago, must have been incandescent / fairly hot).
    – user416
    Dec 3, 2013 at 7:49

2 Answers 2

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It is actually not uncommon for fish to disappear, seemingly without a trace. It has happened to me a few times. Sometimes you eventually find out what happened, sometimes you don't. There are several things that could have happened to it. Each of these things has happened to my fish at some point in the past.

  1. It might be hiding. Some fish are really good at this. We had a kuhli loach that was very good at hiding in the gravel. One time we didn't see him for a week, and we thought sure he was gone, and then one day he was swimming around the tank again.

  2. It could have jumped out. Even if you have a cover on, if there is a small gap around the filter or heater cord, a fish can jump through it. We had a glass bloodfin tetra that disappeared, and we eventually found it on the floor underneath a book rack next to the tank.

  3. When a fish dies, often other fish will eat it very quickly. This can happen fast (a matter of hours). It doesn't mean that you have aggressive fish, it just means that once your fish dies, the others see it as food. They are fish, that's what they do. This is probably the most likely scenario in your situation.

  4. On the other hand, the fish might be the victim of an aggressive fish. This is always a possibility, even if the fish are supposed to be peaceful, and there isn't a big size mismatch. We had a molly that, for whatever reason, always attacked any other molly in the same tank.

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    I really like this answer, it some of these things would never have occurred to me. And as for the other fish eating a dead one so quickly, it's been a long time since I've seen that. Our neighbor once put some carp in with our goldfish and the carp ate all our goldfish, I can't remember how it happened, I think I have blocked it our of my memory! seriously..
    – user6796
    Dec 3, 2013 at 9:43
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Some thing that happened once was our smallest goldfish jumping in the filter and being stuck >_<'

Thankfully we cut the pump and took it out with a fishnet

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