It's been a while since I've caught one, I can't remember if it swims faster than the Frost Dekoi or slower than the Icecuda. I think it swims faster though. If you don't have Winnow yet, scare away all the non-Ice fish and study their movements. If they all seem to be moving relatively the same speed, do a summon spell.
I could be wrong, but I do believe the Cranky Catfish swims faster, and the Owl-Eyed Pike swims slower. But that's my tip is to pay attention to the speed they swim. You'll save energy and time if you only catch the fish that show anomalies rather than catching all of them.
And yes, they are only ever in the main pond. It's possible they sometimes spawn in the one over by the Death school, but I have never looked for or found them there. I've also never found them in the pond by the Pet Pavilion... only Frost Dekois and Icecudas over there, along with the occasional seasonal fish.