if you want it to appear on 3 seperate lines, just use
Code:
str_for_num (message_text.string[1], fish_counter);
to update the second string with the fish_counter

if you want it all on one line use,
string message = "#30"; //set a string to hold 30 characters
Code:
str_cpy(message, "You have ");
str_cat_num(message, "%.0f", fish_counter);
str_cat(message, " fish!");


when converting a data type to a string, you need to place it into a string
for the font size, look at text.font in the manual

Hope this helps