I have succesfully amplified gouramy with universal COI primer with annealing temperature done at 55 but in other fish the temperature work at 50. what do you think could happen?
Why can certain COI fish amplify at high temperatures (55) while others at lower temperatures?
If there are some mismatches between the primer and the target, the primer will melt at a lower temperature. It's likely the sequence in gourami is exactly the same or very close to the sequence of the primer, which is why you got amplification at 55C. If the other species you tried have a few mismatches, lowering the annealing temp worked by helping the primer bind despite the mismatches.
I agree with the Laura's answer,
The primers interact with the template during the annealing step of the PCR reaction. A partial match between the primer and the template will be stable enough at lower temperatures, resulting in amplification from more locations. The higher the temperature, the longer the compatible sequence the primer must bind to, and thus the higher the specificity.