I bought 2 of the red this past Saturday and seriously think it was a mistake. I have not seen either of them since that night I put them in. I think I have an idea of where they're holed up but these last few days I have not seen them even poke their heads out or coming out to eat.
Fire fish can be shy, but once they feel secure, they come out more. They can hide for a long time. Just make sure to send some food close to where they are hiding, so that they do not starve. I got lucky as they were very small when I bought them, but I got a male and female, they paired and I saw them spawn once. The male is slightly bigger and slightly more colorful, but you need to see both side by side to tell the difference.
If you want a group, the scissor tail are a better choice, and I find them a lot less shy.