There are such sentences as below. It seems that all of them are grammatical.  Who could explain to me which is correct and which is wrong? Thanks in advance.

1. When he realized that this was no ordinary fish, he tried not to damage it.

2. When he had realized that this was no ordinary fish, he tried not to damage it.

3. Hading realized that this was no ordinary fish, he tried not to damage it.

4. Had realized that this was no ordinary fish, he tried not to damage it.

5. Realized that this was no ordianry fish, he tried not to damage it.

6. Realizing that this was no ordianry fish, he tried not to damage it.