How do I effectively rub spices into fish?
First of all, if you're seasoning tilapia, you'll want to add some oil to it, since tilapia has almost no fat.  So, here's the steps:


Drizzle oil over the tilapia (both sides).
Sprinkle it lightly with the spice mixture, all over
Let sit 10-15 minutes.
Sprinkle with starch (e.g. flour) at this point if you're frying them.


Optionally, you can also add some lemon or lime juice (just a little) in step 1.  You also want the tilapia sitting on something flat, with the filets in one layer, like a large plate or baking sheet, not heaped up in a bowl.

For spicing small filets, you want a finely ground, moderate-flavor spice mixture.  Unlike beef or pork, you're not marinating the fish for a long time, and you're not cooking it for  along time.  So the spicing on the fish isn't really a "rub"; it's spicing to eat.  I recommend against using rubs designed for meat on light fish like tilapia.