It's side to side where I'm having the problem. They're just a fraction on an inch too small to fit securely. The front to back measurement is not as critical. Would that fix still apply?
I forgot to ask if your 75 gal tank has a center brace? If so, then see options below. Also, do you have any idea the brand of your tank?
Both recommendations are 'fixes'. If you mean tile 'backer board', not tile board, then I guess that would be another option. I don't think the tile or backer board would be the aesthetically the best fix option though. So now I guess you have four options.
* Return the canopy and get a new one that may fit better (time and shipment $)
* Have a new rear glass piece cut a bit wider (Some extra time and some $)
* Use tile or backer board to fill the gap (Some DYI and the least aesthetically option)
* Cut some corners from the clear plastic gap plate and position them so they are slightly extended in order to rest on trim (easiest option with no extra cost)