wow this is sweet, cant wait to see how you finish it off. I am currently thinking of getting 2 large tanks but I am crap with wood construction so this gives me hope! I was looking at cement also but like you mentioned you cant move it.
Your CNC machine is also very nice hehe, I have 3 phase in my fish room all ways thought about extending a bit for something special... you really have all the boy toys!
Do you think your method would work using glass panels instead of acrylic? also before bolting the panel to the frame I imagine some type of silicone would be required?
As for tig welding well I think its worth trying to learn I myself cant do it but everyone at work says once acquired its much easier and faster.