Hi Karan,

There two was through which you can achieve this requirement. Either write a custom Save Button on Case and create records there or  you need to create a trigger on Case, which would fire when a record in inserted and would generate 6 records on a related object is Contaminants__c.

I have written a sample code for you. Don't forget to check the api names before saving your code!

Follow these steps:

Step 1: Go to Setup and in the Quick Find Box, type Classes.
Step 2: Select Apex Classes
Step 3: Paste the following class in it.
 

public class CaseTriggerHelper{
	// Static map to store your Contaminant Name
	public static map<integer,string> mapIntToContaminant = new map<integer,string>{0=>'oxygen',1=>'CO',2=>'H2',3=>'Olefins',4=>'Sulfur  components',5=>'Chloride components'};
	
	public void createContaminats(List <Case> cases){	
		
		// We need to store a List of contaminants to create.
		List<contaminants__c> contaminantsToCreate = new List<contaminants__c>();

        // Loop over the cases. Remember, we don't know how many cases we will have.
        for(Case eachCase : cases){
			for(integer i =0; i <6; i++){	
				contaminants__c newContaminant = new contaminants__c();
				contaminants__c.Name = mapIntToContaminant.get(i);
				contaminants__c.Feed__c = False;
				contaminants__c.Regeneration_medium__c = False;
				contaminantsToCreate.add(newContaminant);
			}
        }
		// Insert your List
	    insert contaminantsToCreate;
	}
}

Step 4: Go to setup and in the Quick Find Box, type Case.
Step 5: Select Case Triggers.
Step 6: Paste the following Trigger in it.

trigger CaseTrigger on Case (after insert) {
    // Create an instance of your helper class
   CaseTriggerHelper helper = new CaseTriggerHelper();

    // This says run if the trigger is after the insert
    if(Trigger.isAfter && Trigger.isInsert){
        // Now call your helper method
        helper.createContaminats(Trigger.new);
    }
}

Step 7: Check if it works or not :)

Can you take a moment to  upvote and mark this answer as solved, if it helped you.
Cheers!
Ajinkya Deshmukh