Take a look at the Deltec 600mce this is a hang on skimmer that is good with small gaps check the dimensions though to make sure it will fit another option would be 2 mce300.

There aren't any internal skimmer on the market that are up to the job I'm afraid unless you add multiple internal nano skimmers.

A hang on Deltec is your best bet take a look at them get the dimensions and hopefully you will be able to accomdate one of these if not seriously explore moving your tank an inch or 2 further from the wall.

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Alan

Thweeerp!