Lost in the loops!

[Redeyery]

I'm attempting to make a text-art fish tank that can hold multiple text-art fish in random positions. The displayTank() function works well other than the fact that the new fish replaces the latter. Opposed to having two or more fish in the tank, I only have one. How do I replace the empty tank with a more fish rather than replacing the old fish with the new one?

    //Uses a vector to store different parts of the tank.
    void displayTank()
    {
        tank_frame.resize(10);
        //Build tank
        for ( int j = 0; j < 10; j++ )
        {
            //Top
            if ( j == 0 )
            {
                string top;
                top = ' ';
                for ( int i = 1; i < tank_size; i++ )
                {
                    top += '_';
                }
                tank_frame[j] = top;
            }
            //Middle
            else if ( j > 0 && j < 9 )
            {
                string middle;
                middle = '|';
                for ( int i = 1; i < tank_size; i++ )
                {
                    middle += ' ';
                }
                middle += '|';
                tank_frame[j] = middle;
            }
            //Bottom
            else
            {
                string bottom;
                bottom = '|';
                for ( int i = 1; i < tank_size; i++ )
                {
                    bottom += '_';
                }
                bottom += '|';
                tank_frame[j] = bottom;
            }
        }

        //Generate random numbers for fish positions in vector
        if ( fish_collection.size() != 0 )
        {
            vector <int> randHeight;
            vector <int> randWidth;

            randHeight.resize(fish_collection.size());
            randWidth.resize(fish_collection.size());

            for ( int i = 0; i < fish_collection.size(); i++ )
            {
                srand (time(NULL));
                randHeight[i] = rand() % 10 + 1;
                randWidth[i] = rand() % (tank_size - 5) + 1;
            }

            for ( int j = 0; j < fish_collection.size(); j++ )
            {
                tank_frame[randHeight[j]].replace ( randWidth[j],                    fish_collection[j].size(), fish_collection[j] );
            }
        }

        //Print the vector
        for ( int j = 0; j < tank_frame.size(); j++ )
        {
            cout << tank_frame[j];
            cout << endl;
        }
    }

    void newFish()
    {
        string input;
        cout << "Create new fish (up to 5 characters long):\n\n";
        cin >> input;
        if ( input.size() > 5 )
        {
            cout << "Fish is too big!\n\n";
        }
        else
        {
            fish_collection.push_back(input);
        }
    }