Yes, fish do have ears inside their heads, but many do not hear sounds very well. They use what are called "latteral lines" that run along the sides of the fish. They use these to sense changes in water pressure. This is similar to the way humans ears sense atmospheric pressure changes.
The lateral line (note spelling) is not used for pressure detection, but rather for detecting water motions around found around fish (within a body length or so) up to about 100 Hz. The lateral line has nothing whatsoever in common with human pressure change detection - the receptors are different and atmospheric pressure changes are very low frequency (below 1 Hz).
The ears of many fishes are able to detect sound (particle motion) and while many species do not hear as well as humans or other mammals in air, they are sensitive enough to be able to detect the "acoustic scene," which means that fish can hear the sounds around them of biological relevance. Indeed, hearing in vertebrates evolved (in fishes) to give the animals information about their extended world, far beyond the range of vision. In effect, by using hearing, fishes (and all vertebrates) get a 3 dimensional "view" of the world around them that extends far beyond the range of other senses. This "acoustic scene"works in day and night, and enables animals to be aware of their environment.
If you mean freshwater sunfish family, the members can grow up to 15 inches.
The Oscar fish naturally stay calm when they are in a pair.keeping more than two Oscars in the same fish tank can cause then to get aggressiveso keep two Oscars at the max in one tank.
Portuguese man o'war (Physalia physalis) is not a jellyfish, however it produces a liquid mixture made up of three peptides:
* hypnotoxine, a neurotoxic poison, which arrests respiratory, circulatory and nervous systems; * thalaxine and * congestine, two allergens, which produce irritation and kill by anaphylactic shock.
that is mostly a sign of a common disease known as ick [ tiny white spots on the body if not treated soon fish will die occasionally. ] treatment for this is ick away, ick guard, or something that has to do along the lines of that. if you need help ask someone at the pet store the might be able to tell you. but however there is other common disease that can cause that. im not sure what can but the only one that i know of is ick.