My general thought with most engineers is that they are more than happy to spend somebody else's money - needed or not. Has anybody ever met one that said "you are good?" Probably, but I have rarely met one. If anybody finds one of these good ones, then let me know.
You can probably do most of this yourself. Figure out the type of joist and span and google the load. There are differences between live and dead load. If you are close, then sister it up or add another one. If it is an engineered joist, then this is a bit harder, but still not impossible.
I had a friend who was right at the limit of a single 12 inch I joist. He could not get a 12" to slide in there, but got an 10" right next to it and then blocked it up flush to the floor and has more than enough weight bearing range now.
Any good GC should be able to handle a sistering, even if there is ductwork or electrical there.
Tank of about 12-15 pounds per gallon is an ok rough estimate... but nothing is perfect unless you weigh each piece (I like 600lb of live rock and you like 200lb, for example). Larger tanks weigh more, even by the gallon.
I agree with Syn, if this is less than a 180g, then you are likely fine. If you are talking like 350-400+, then I would nearly always put these on concrete regardless of joist and subfloor situation unless it was built just for this.