Sometimes when the water wanders a little from parameters, skittish fish will get a little more so. They "want" to hide. I had some killifish (some sort of Epiplatys, I seem to have repressed the memory) try and get down the stem of a plastic box filter. I removed 3 of those idiots, fortunately while they were still alive. The tank was given a partial water change. The next day I fished a 3-inch/ 7cm corpse from that very same filter stem. Sigh!

It is odd that no body was found the day after he disappeared. It is possible that the barb killed him, but there are a lot of other possible reasons for his death. Unless there is an oxygen shortage, it is often the male guppies that are first vulnerable to increased bacteria levels in the water.

You certainly were wise to remove the barb. Though I've only had a few, they are not good company for guppies. In a community tank of medium sized tetras and barbs they do well if in a small school of their own species. (For some reason folks often recommend six or more.)

It is an uncomfortable thought, but aquarium fish do sometimes cannibalize dead tank mates. Male guppies, massing less than females, will break down biologically quite quickly anyway. If the barb and other guppies ate most of the body, the rest could have been mostly broken down, although it does seem unlikely.

If you had several pond snails or one large snail in the tank, then the fate of the body is not such a mystery.

We frequently recommend partial water changes (and maybe a more frequent pattern of partial water changes increasing the amount of treated, seasoned water from 25% to 30% to 35% to 40% to 45% at a time. That may sound blissfully ignorent.

However more fish pathogens than we realize may be present in a fish tank, all or much of the time, but at a non-lethal level. All of us get busy, especially from time to time and often water quality declines, weakening the immune systems of fishes.

It is good to enumerate the things in the water we should look for and even test. However, those in the hobby keep expanding our knowledge of what we need to consider. When I started about 200 years ago <G> about the only thing people measured for was pH, which is usually the least of our problems. The only water condition we looked for was, "was it dirty or smelly?"

Today we know to monitor the nitrogen cycle and measure for ammonia, nitrite and nitrates. High and low pH, TDS, KH, DH and a host of other things are considered. However, in light of all of the new discoveries on water quality recently, I would be surprised if aquarists, a few years or decades from now, were not testing for more.

So in the meantime, we know that water tends to deteriorate in quality. So we change some and then some more. Many fish bounce back.

Students also are more vulnerable to a variety of illness at the end of the term or semester. Since so many of those students (and many of us back in the Middle Ages) had burned the candle too long, at both ends, that "get a couple good meals under your belt and a couple good night's sleep" again sounds awfully simplistic. But it helps. :)

Don't apologize for the length of your post. The more info the better! Readers better understand your aquarium's or fish's situation. Sometimes in detailing the case, the writer also discovers details, which help lead to greater understanding.

This submission is something of a log and something of a diary. Diaries tend to be shorter (at least that was the site's software designer's plan) but they are immediately posted to the "diary" section and appear on the Guppylog front page if one clicks on "everything." Response tend to be quicker. It may simply be that many people start on the left and only get to the Moderate Log Entry Submissions section if they still have time.

Out of curiosity, are any guppy fry developing into males with that blue pattern which you so liked? If he was shy, he may not have inseminated any of the females or as effectively inseminated them as the other males. (In fact, researchers have found that if a male had an illness, the female will tend to avoid him and his advances.) But it would be nice to see some of his offspring show up. :)

Welcome to Guppylog! Thank you for sharing "The case of the fish that disappeared." Please bear in mind that I could be entirely wrong about why the deceased vanished though. :)