To use "removeMovieClip();" the object must have a depth. Depth is assigned to an object when it is either duplicated, or manually assigned one, by using "this.swapDepths(#);"
Once the object has a depth, to remove it, you simple put this code on it:
this.removeMovieClip();
or.. You can do it from outside the MC, and say:
_root["fish"+i].removeMovieClip();
Or something similar.
If I wasn't clear on that swapDepths thing, here's how that works:
this.swapDepths(100);
this.removeMovieClip();
If you put that on an object, it gives the object a depth of 100 over everything else, or possibly under anything else with a depth over 100. Then, the code removes itself, deleteing it and it's assigned depth.