Welcome To MR.
I kept FW tanks before SW Also. As most of us here probably did.
When I first started thinking of Starting a SW tank. I wanted Tangs also.
But after having them. In tanks that were just too small. I ran into problems. Aggression prolems.
Tangs can be pretty territorial. Problem is. Their territory is Everywhere.
There are alot of great fish you can house in a 40breeder.
I like Blennies,, gobies,, Hawkfish. Stuff like that. But most would tell me Hawkfish will kill my shrimp and crabs. But they don't.
So you never know. LOL