Maybe it has never happened to you, or maybe it has, and you have wondered what happened to that fish that perhaps was the one you liked the most or one of the ones you had booked due to its physique, but, overnight it has disappeared. What happened to him?
There are times, because fish are like that, that they hide in the "toys" they have in the aquarium or in the plants, but when you spend a while looking for it, even moving those objects in the aquarium and you can't find it, you start to worry.
Various theories can be given for these types of situations. The first is that the fish has been attacked and eaten by others. It can happen, it is common and sometimes it is because you do not add too much food so, at night, they tend to attack each other to feed on the weakest.
Another possibility, one that many times we do not realize, is that the fish has entered where the heater and the pump are. It has happened to me, in fact it happens to me right now, with a fish that grew up in there and every time I take it out it gets alone (I don't know how) and disappears two days later.
This is very difficult to remove if you do not empty the aquarium and many fish prefer this in order to be more calm (they become not sociable). However, there may be a problem if you have an aquarium with a compartment for the pump and heater and, between that compartment and the aquarium, there is a small space. Why? Because there you can put the fish too.
As a rule the fish would not get in (I say as a rule because my fish is in there). Taking them out is difficult, not impossible, but when you see that he does the same when you try to get him to be with others, it complicates things.
Leaving it inside the compartment or between it and the aquarium causes it to become antisocial and not grow as it should, but being with other fish makes it very nervous and that may be the reason why it gets lost.