I have a few recipes for salmon, and a freezer full of tilapia. Can I just use the tilapia instead of salmon? It's for things like "salmon and rosemary", and "poached salmon with cucumber dill sauce" for example. I know fish is not all the same, but I would like to try these out without having to get more fish when I have so much. So can I just change it right over, or will it not taste right?
It depends on the recipe.
As Jefromi said, salmon does have a different flavor. Specifically it has flavor. Tilapia doesn't really have much flavor at all.
More importantly, salmon has about three times as much fat as tilapia. Salmon holds together better than more fragile white fish. This makes grilling salmon much easier than other fish which fall apart.
If your recipe highlights the flavor of the fish, or relies on the fat or cohesion of salmon then you should not make the substitution.
On the other hand if your recipe calls for baking in a pan and uses other flavorings, such as lemon, that can overpower the flavor of the fish, it would be a good candidate for substitution.
Salmon has a bit of a distinctive flavor; recipes meant for it probably won't match a mild white fish like tilapia quite as well. You probably want to look for recipes for more similar fish, like catfish, red snapper, bass, or sole.
I would use wolf herring to replace salmon. They have nearly the same texture, in that as a kid I could peel the flesh of poached steak off into layers for either of them. Wolf herring flesh is similarly as stable as salmon flesh.
When you cut up wolf herring and salmon into steaks, and take a b & w picture of them, they are nearly indistinguishable in shape and texture.
Both of them can be made into fish gefilte that hold together reasonably well (after doping them with tapioca flour starch). I have made fish balls out of both of them with similar results. They have the same firmness.
Their similar firmness allow both of them to be similarly poached or grilled without their flesh falling apart. You could similarly drop both their grilled steak from a height of 8 inches onto the table/plate and they would not disintegrate.
Wolf herring also has a rich and amenable flavour as does salmon.
Wolf herring is about the same body size and length of salmon.