Student or Learner
"if i was a fish, i wouldn't get caught."
what tense is this? and how to change it to past perfect tense?
It does make sense in the past perfect. Used as part of a Third Conditional sentence it expresses an unreal or hypothetical situation. It is often used to express regrets for something you didn't do in the past or didn't happen in the past.
If I had studied Spanish when I was young, I would have been fluent by now.
I I hadn't eaten so much, I would not have become fat.
These are considered unreal or hypothetical because you are talking about the past - which we cannot change. Your example is a little more unreal than mine.
Hope this helps.