#index docs decreasing after adding/starting crawler (jobs)

(from github.com/zopyx)
I had about 1.8 million docs indexed. I added two more web crawlers and related crawl jobs. Checking the dashboard revealed that the number of indexed docs went down to 600.000 items…what is happening here?

(from github.com/marevol)
In General page, you can set document TTL at Remove Documents Before field.
If removing TTL, set it to -1.

(from github.com/attibalazs)
Hi, i checked the help for the ,“Remove Documents Before” field i`m not exactly sure what this means.
The help states only: “Days which old documents are removed.” Can you provide more detail please?

(from github.com/marevol)
It’s TTL for documents.
If indexed documents is not updated, they are removed.