Chloramine is produced when a water treatment plant injects both chlorine and ammonia
simultaneously into the water supply.

This is done to reduce the free chlorine's ability to combine
with organic matter in the water and thereby produce potentially
harmful trihalomethanes, or THMs.

Because chloramine has less disinfecting ability than free chlorine,
a higher concentration of chlorine is usually used to get the same bacteria kill rate.

Chloramine Toxic to fish
Chloramine is very toxic to fish.
Chloramine, unlike chlorine, passes through the gill tissue causing little damage.
Chloramine then enters the bloodstream and binds to iron in the hemoglobin in the red blood cells.
The blood is unable to carry oxygen and the fish become
tired, inactive and sits on the bottom or gasping near the surface.

Removal?

Un-informed sellers of Shower filters are claiming that they remove chloramines
and leads and heavy metals because they use carbon or carbon and KDF media.
While KDF may have some ability to remove these contaminants
however in the small amount of filtering media and the rapid
flow rate of a shower filter, this is unlikely.
If you are using this kind of shower filter, you should
not assume it will be safe to add water directly from it
to your aquarium. However, using these media in your
aquarium filter can filter away chloramin because the water
is repeatedly passed through the filter unlike the "one-pass"
of the shower filter.

If you are unsure about the chloramine content in your water,
you can call your water supplier for this information.
If your water contains chloramine, treat the water before you add
it to your aquarium because it can damage your biological filter
by killing off beneficial bacteria in a cycled tank.