I am looking for a very certain type of warm freshwater aquarium fish.
I used to own some a long time ago, but I can't recall their
scientific name. I've been searching for a picture all over the place,
but couldn't find one, thus I'll try to describe them as good a
possilbe.
The are about 2-4 inches long, long and skinny, and most notably, can
be "seen through". Their skin and flesh is not colored, instead you
can see their skelton. I recall that they are considerably rare, as I
used to pay about $10 a piece 10 years ago... they are probably $25
nowadays...

What I'd like to see is a picture together with the scientific name.
The picture is key, I recall how they look, but not what they are
called.

Dear treo,

Following your description, I am quite sure that the fish you have in
mind is the Glass Catfish, also known as Asian Glass Catfish, Ghost
Catfish, Phantom Catfish or Indischer Glaswels. Its scientific name is
"Kryptopterus minor". The adult Glass Catfish has a size of about
2.75" to 3.2".

"Kryptopterus minor" is a rather recently coined term. For a long
time, this species has been misidentified as belonging to the related
species "Kryptopterus bicirrhis" (which is called Glass Catfish, too),
another kind of transparent fish that looks very similar, but grows
considerably larger (up to 6" and even longer somtimes). The
transparent African Glass Catfish, or "Parailia  pellucida" from the
family of Schilbeidae catfish, is also sometimes confused with
Kryptopterus minor, although the adult African Glass Catfish is about
6" long.

Kryptopterus minor is indeed transparent as glass, thus revealing its
skeleton and internal organs in a most amazing way. Please follow
these links to see photographs of that fish:

Zoopet.com: Kryptopterus minor
http://www.zoopet.com/akvarium/fisk_bild.asp?BILD=231

A.V. AquaVISie: Fish and Plant Database - Kryptopterus minor
http://members.brabant.chello.nl/~j.bukkems/Vissen/Kryptopterus_minor.jpg


Sources:

Pet Resources: Fish Profiles - Glass Catfish (Kryptopterus minor)
http://www.petresources.net/fish/catfish/kry_bic.html

Planet Catfish: Kryptopterus minor
http://www.planetcatfish.com/catelog/silurida/kryptopt/16_f.php

Osteichthyes - Transparent Fish
http://oceanlink.island.net/ask/osteich.html#anchor252388

FishBase: Parailia pellucida Species Summary
http://www.fishbase.org/Summary/SpeciesSummary.cfm?ID=9372&genusname=Parailia&speciesname=pellucida

California Academy of Science: West African Rainforest Fishes
http://www.calacademy.org/science_now/archive/wild_lives/af_fish_053101.html

Planet Catfish: Parailia pellucida
http://www.planetcatfish.com/catelog/schilbei/parailia/3_f.php

FishInThe.Net Community: Glass Catfish
http://fishinthe.net/html/forum/viewtopic.php?t=2534

Search terms used:
"glass catfish"
://www.google.de/search?q=%22glass%20catfish%22&hl=de&lr=&newwindow=1&sa=N&tab=iw
"durchsichtiger fisch"
://www.google.de/search?hl=de&newwindow=1&q=%22durchsichtiger+fisch%22&btnG=Suche&meta=
"transparent fish"
://www.google.de/search?hl=de&newwindow=1&q=%22transparent+fish%22&btnG=Suche&meta=
"Kryptopterus bicirrhis"
://www.google.de/search?q=%22Kryptopterus%20bicirrhis%22&hl=de&lr=&newwindow=1&sa=N&tab=iw
"glass catfish" Schilbeidae Kryptopterus
://www.google.de/search?q=%22glass+catfish%22+Schilbeidae+Kryptopterus&hl=de&lr=&newwindow=1&start=0&sa=N
"Kryptopterus minor"
://www.google.de/search?q=%22Kryptopterus%20minor%22&hl=de&lr=&newwindow=1&sa=N&tab=iw

Hope this is what you were looking for!
Regards,
Scriptor