If I'm not mistaken, the 16mm is the internal diameter of the connection hose of the heater, so the out side will be around the 18mm mark, I can't confirm this for sure tho, my heater is hooked up and I can't check for you, but tomorrow I will check the box and see if I'm right and I will post back.
Also, if you did decide to order and it was a little loose, you can use a metal thread tie, I normally use these anyway ( for peace of mind )
I have fit the 5/8" (16mm) 300 watt into 1/2" (12mm) hose. just a 1/8" or 3mm difference.
It took some soaking of the tubing in hot water to soften the hose and some working, but it worked out fine.
The smaller however might not work out on larger hose and could be prone to leaking.
Edit: I looked at the operations manual My link
and it is a off size hose that's for sure. Justin may be right but the 25mm (.98 inch) outside concerns me as the screw down locks should fit slightly over the end of the hose.
i use a hydor inline 300w attached to a tertatec filter external, both came recommended as use same size piping and they are AMAZING, i have a perfect temp in my tank on both sides as i use a digital thermometre