Well, as you know, the kibble itself really isn't biologically appropriate, grains or no grains. So is fish worse than vegetable matter or the highly cooked kibble? Don't really know. Haven't seen a lot of writing on this.
In an ideal scenario, I would avoid fish except for perhaps occasional treats. In my current scenario, I feed fish from cans, and the dry food I'm still feeding has very little to no fish meal in it. The Nature's Logic does have white fish meal in it, which is a little more descriptive than usual, but still could be from various fish.
Current morning meal for the last few days has been Tiki Cat tuna with chicken (which actually has a fair amount of chicken, unlike some such blends) laced with some extra salmon also from Tiki. See? Lots of fish.
The wild-caught salmon is typically not a heavy metal concern at least. When I put a lot of chicken in the bowl, though, it still didn't work out well the other day (even with the salmon mixed in there) so I'm unsure of how far I can push it. The Halo Dinner Party topper is helping a little, I think, maybe. The bowls are coming up empty the last few days, which even with the fairly small amounts wasn't always happening before even with similar foods, even when it was all fish.