2 rubber seals on the 2 trays at the pipe where you are suppose to align when putting the trays together. Mine comes installed but you should see it when filling the trays with media especially the blue or white sponge.
Not too sure on the strainer. I will leave it on like you mentioned to prevent fishes and gravel being sucked in. Other options will include those glass inlet pipes or Eheim Installation Kit 1/2. But I don't think any of those will increase the sucton of fish poo. Best is to ensure adequate flow in your aquarium.
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