I think we need to take a step back on suggestions and ask the OP if they know for SURE what tank they will be running. Trying to buy a canister that will suffice for both a 40gal and a 125gal tank is not a good idea and not the ideal way to handle things. Buying a canister filter for a 40gal and a canister filter for a 125gal is the least expensive and best way to go about this situation. Running a FX5 for a 30gal is overkill, plain and simple. Keeping it turned all the way down puts more wear and tear on the circulation system and it's better to just allow them to run w/o back pressure to lower speeds. Not to mention it costs WAY more in the filter and media than you need for a tank that size. On top of all of that you have electricity use and then the real estate for a filter that size may be difficult to come by under a 40gal. These are all practical considerations and if you weigh them all carefully I think you will find that having a canister filter capable of running a 125gal aquarium to be a square peg & round hole scenario. There are a LOT of smaller filters that would be perfect for that 40gal aquarium that I would consider if I were you and if you end up needing one for that 125gal then you can shop for filters capable of handling that. Heck, I've got a lightly used Fluval 305 that I'll be selling in the next couple of weeks that would be perfect for a tank that size and save a TON of money over an FX5 in electricity, media and initial cost. Not trying to peddle my own wares but giving you an idea what would be out there that might suit your needs.