You must have a different crock pot than me. Mine only heats on the bottom, as the crock is removable for serving and cleaning, and the side walls are thin metal - structural only. So, it naturally has a temperature gradient from bottom to top. If your's heats the walls as well as the bottom, I can easily see how it works well for you. A buddy of mine has a similar crock pot to me and tried the PID controller. Didn't like the results and went out and got the Anova. Loves it.
BTW - I've slow poached shrimp and sea bass in butter using the water displacement method in a large Ziploc bag with the Anova. Works great. Eager to hear how your fish comes out. I'm looking for an excuse to splurge on some lobster tails and try those!