How to add an if statement for one of the custom fields on the following loop?

I have a bunch of custom fields on one of my post types, that is displayed randomly but in a fixed order. The names of the custom fields are always the same, and for every loop there is an increasing counter added to the custom field names.

Like this:

custom_text
custom_text1
custom_text2

custom_image
custom_image1
custom_image2

custom_video
custom_video1
custom_video2

The problem is, there is one field that begins with the number one, like this:

custom_special1
custom_special2
custom_special3

Right now, it's impossible to change this behaviour. Problem is, this makes the custom_special fields show up in the wrong place of the output, the first one shows up together with the fields with the number 1 at then end, instead of with the empty ones.

I need to tweak the code below with some kind of if statement, that says something like:

if $custom_special . $counter {output the result one step down}

Here's my code, I hope some of you gurus out there can help me with a solution!

// max custom field index
$number = 40;
// the counter
$counter = '';
// the meta keys to check for
$keys = array(
    'custom_text',
    'custom_image',
    'custom_video'
);
// all our custom field values
$custom_fields = get_post_custom( get_the_ID() );

// loop over our counter
while( $counter < $number ){
    // loop over each of the keys
    foreach( $keys as $key ){
        // check if a custom field with key + counter exists
        if( isset( $custom_fields[ $key . $counter ] ) ){
            // output the field
            // values will be in an array, 0 is the first index.
            // you can loop over these as well if you have multiple values.
            echo $custom_fields[ $key . $counter ][0];
        }
    }
    // increment the counter
    $counter++;
}

Thanks a lot!

// Jens.