Missisboot tisn't true I'm afraid. Butter and oil can both get equally hot.
Oil is better for crispy skin because butter contains water and milk solids and therefore makes the skin soggy/ burns.
For crispy skin you need to pat the fish skin dry on some kitchen paper, add a glug of oil to a medium-hot pan then place skin side down. To get the skin evenly crispy all over, press the fish down with a fish slice straightaway. Keep pressed down for a minute until it stops curling up.
It's better to go medium-hot for a bit longer than really f-ing hot for a couple of minutes so the skin has time to crisp up all the way through without turning black. You can cook the fish most of the way through with the skin side down then just turn last minute to finish off.
Am a food geek, apologies if you've now lost the will to live let alone fry fish