If all you have is a whisper filter on a 75 then as you most likely guess this is really under powered, under filtered tank. I would hope that maybe you also have a few power heads for added circulation. (please give me a clue if you do.)
IMO/IME - your first step in better filtration will be that of Live rock (do you have any?_) and Live sand (Got any_?)
Those two things will go a long way to providing you with a great natural filtration, as long as you have some good water circulation.
One other idea to improve your filtration is to add an external overlow box that leads to a sump or another tank, the water might go through a filter floss or just through an in sump skimmer, then back to the main via return pump. Just an idea.
To answer your question directly. I would have to say that even if you were to add a single canister filter then you will still be under filtered. Most canisters that I have seen or used are under powered, and a maintenance pain. THe canister will not provide you with any surface agitation for oxygen exchange. A single canister on a 75 would not solve your filtration problems but would help.
If it were a choice between a skimmer and a canister I would take the skimmer first.
Thomas
BTW - Welcome to the Board :happy: