rma says that those who don’t eat hot food are apikursim…
Yes, but that’s because they are showing with their actions* that they don’t hold of TSBP. I don’t think the same applies here with fish.
The Wolf
* Of course, that only applies if you do so purposely because you believe it’s the halacha. I’ve had cold-only meals on Shabbos (for taste/convenience reasons, especially in the summertime) and that doesn’t make me an apikorus.