I think there are several key things to crispy fish.
I do not think the coating you choose is critical. I have tried many coatings with good results.
The main thing, like many have said, is
maintaining 350 to 375 degree oil.
Don't overload the fryer. If you dump a whole bunch of cold fillets into your hot oil the temp is gonna fall like a rock.
This is what you want to avoid.
If you are only using a small amount of oil, only cook a few fillets at a time. Maybe 3.
The more oil you use, the less the temp will drop when you add the fillets, so you can cook more fillets at a time.
I like to make sure all my pieces of fish are fairly thin so they cook quicker.
If the fillets are from small crappie, they are usually fine as is.
If they are from some big slabs, I will cut the fillet into about 3 pieces.
Plus I cut them on a bias, or at a steep angle.
The idea is to not have any pieces too thick,
so they cook fast.
If you have the right temp, and your pieces are not too big, they should float pretty quick.
When they start floating, they are about done. Watch the bubbles. That's the water cooking out. When the bubbles slow down a little. That's when I declare them done.
Hope this helps.