Note 1: Your constructor parameter names are not very explicit...

Note 2: When you have such long list of parameters, it is useful to split it on several lines:
    f1 = new fish(
        (int)random(TANK_LEFT+randw, TANK_RIGHT-(randw/2)),
        (int)random(WATER_TOP, WATER_BOTTOM -(randh/2)),
        (int)random(randw-20, randw),
        (int)random(30, 50),
        random(0, TWO_PI),
        (int)random(0, 256),
        (int)random(0, 256),
        (int)random(0, 256));
More readable, IMO.

Note 3: If you don't think to differentiate your fishes by other ways than random drawing of numbers, and since all constructor calls are the same, you better pass no parameter at all, and assign to the class fields (its variables) the random expressions directly. It simplifies even more the construction of fishes.

Note 4: " basically saying when the direction variable is between PI*0.5 and PI*1.5, then the object (fish) will have a "left" orientation." " if (f.dir == (random(PI*0.5, PI*1.5)))" " My logic is probably wrong." Indeed...
Well, here that's not the logic that is wrong, but the implementation of the idea.
dir is a random number between 0 and TWO_PI. OK. But you check it against another random number between PI/2 and 3*PI/2. The chance the two random numbers are equal is... very very thin!

The proper check would be:
if (f.dir > PI*0.5 && f.dir < PI*1.5)

Note 5: the function name isn't proper, something like drawFish() would be better.

Note 6: it would be more logical to put this function in the class itself. It is then called a method and it can use the variables of the class (the fields) without prefix.

Note 7: if you want to use your fishes in draw(), you better put the declarations outside of setup().

That's all for now... Nice fishes. >

here....

you have to set f.dir to random value first and then evaluate it in if

I made the relevant lines bold in the code....

1. int randw = (int)random(30, 50);
2. int randh = (int)random(randw-20, randw);
3. class fish {
4.   int x, y;
5.   int h;
6.   int w;
7.   float dir;
8.   int r;
9.   int g;
10.   int b;
11.   fish(int p1, int p2, int p3, int p4, float p5, int p6, int p7, int p8) {
12.     x = p1;
13.     y = p2;
14.     h = p3;
15.     w = p4;
16.     dir = p5;
17.     r = p6;
18.     g = p7;
19.     b = p8;
20.   }
21. }
22. //useful constants for Assigment 2
23. final int TANK_LEFT = 100;  
24. final int TANK_RIGHT = 900;
25. final int WATER_TOP = 300;
26. final int WATER_BOTTOM = 700;
27. void setup() {
28.   fish f1;
29.   fish f2;
30.   fish f3;
31.   fish f4;
32.   fish f5;
33.   size(1000, 800);
34.   //parameters for the class so it varies for each face drawn
35.   f1 = new fish((int)random(TANK_LEFT+randw, TANK_RIGHT-(randw/2)), (int)random(WATER_TOP, WATER_BOTTOM -(randh/2)), (int)random(randw-20, randw), (int)random(30, 50), random(0, TWO_PI), (int)random(0, 256), (int)random(0, 256), (int)random(0, 256));
36.   f2 = new fish((int)random(TANK_LEFT+randw, TANK_RIGHT-(randw/2)), (int)random(WATER_TOP, WATER_BOTTOM -(randh/2)), (int)random(randw-20, randw), (int)random(30, 50), random(0, TWO_PI), (int)random(0, 256), (int)random(0, 256), (int)random(0, 256));
37.   f3 = new fish((int)random(TANK_LEFT+randw, TANK_RIGHT-(randw/2)), (int)random(WATER_TOP, WATER_BOTTOM -(randh/2)), (int)random(randw-20, randw), (int)random(30, 50), random(0, TWO_PI), (int)random(0, 256), (int)random(0, 256), (int)random(0, 256));
38.   f4 = new fish((int)random(TANK_LEFT+randw, TANK_RIGHT-(randw/2)), (int)random(WATER_TOP, WATER_BOTTOM -(randh/2)), (int)random(randw-20, randw), (int)random(30, 50), random(0, TWO_PI), (int)random(0, 256), (int)random(0, 256), (int)random(0, 256));
39.   f5 = new fish((int)random(TANK_LEFT+randw, TANK_RIGHT-(randw/2)), (int)random(WATER_TOP, WATER_BOTTOM -(randh/2)), (int)random(randw-20, randw), (int)random(30, 50), random(0, TWO_PI), (int)random(0, 256), (int)random(0, 256), (int)random(0, 256));
40.   makeFish(f1);
41.   makeFish(f2);
42.   makeFish(f3);
43.   makeFish(f4);
44.   makeFish(f5);
45. }
46. /*drawFish draws a fish based on the provided parameters.
47.  Parameters:
48.  int x - x coordinate of fish (centre of body)
49.  int y - y coordinate of fish
50.  int w - width of fish's body (tail is w/2 more)
51.  int h - height of fish's body and tail
52.  int dir - 0 = right, 1 = left
53.  int r,g,b - fill colour of fish
54.  */
55. void makeFish(fish f) {
56.   stroke(0);
57.   fill(f.r, f.g, f.b);
58.   ellipse(f.x, f.y, f.w, f.h); //body
59.   int tail; //x-offset for tail's connection to the body
60.   f.dir = (random(PI*0.0, PI*2));
61.   if ( f.dir > PI*0.5 && f.dir < PI*1.5 ) { //facing right
62.     tail = -(f.w/2); //tail is to the left of the body
63.     //draw an eye on the right side of the head
64.     fill(255);
65.     ellipse(f.x+(f.w*.25), f.y-(f.h*.25), 10, 10);
66.     fill(0);
67.     ellipse(f.x+(f.w*.25), f.y-(f.h*.25), 5, 5);
68.   }
69.   else { //facing left
70.     tail = (f.w/2); //tail is to the right of the body
71.     //draw an eye on the left side of the head
72.     fill(255);
73.     ellipse(f.x-(f.w*.25), f.y-(f.h*.25), 10, 10);
74.     fill(0);
75.     ellipse(f.x-(f.w*.25), f.y-(f.h*.25), 5, 5);
76.   }
77.   //draw the tail using the offset calculated above
78.   fill(f.r, f.g, f.b);
79.   triangle(f.x+tail, f.y, (f.x+(2*tail)), (f.y+(f.h/2)), (f.x+(2*tail)), (f.y-(f.h/2)));
80. }

Hey thanks alot guys that really helped me. (Same guy as kramer733)

that's great!