Question:
After I cooked fish I mistakenly put the cover from the fish pot on a pot of hot soup for a few seconds
Answer:
Common practise is that people cook fish in a meaty pot, even though they are meticulous regarding cooking meat and fish together. There is even more room to be lenient if the fish pot cover had not been used for 24 hours.
Sources:
Y:D 95:Taz 3, Pischei Teshuvah 105:8, Teshuvas Elef Lecho Shlomo 312-313, Chochmas Adom 68:1, Kaf Hachayim Y:D 116:20