The 6" will work with the Sigma
Here's one shot taken with the Aquatica 5dmk2 and the 6" Dome and the Sigma 15mm fisheye
and just for chuckles, here's one taken with the Aquatica 5dmk3 housing, Minidome and the Tokina 10-17 fisheye
Wow! Nice Pictures. Thanks for your answer, now it´s time to buy me a 6" dome.