Do you know whether you have Kryptopterus minor or Kryptopterus bicirrhis? Water stats such as nitrates, pH, temperature, etc. would help a lot. How long have you had them? Glass catfish tend to turn a milky white not the usual clear color when stressed, I have never heard of yellow. Many things could have stressed them out such as being in a trio (they really need at least 8 in a group) poor water conditions, lack of food. Have you had trouble feeding them, you probably do if they are in a community tank. By any chance could you snap a picture of it?