Yes a 45 will work as a sump you will just
NOT have the room to place xtra rock and or have a refugium built in as your filter bag/media, skimmer and pump will most likely fill up all avaialable real estate once you put baffles in to reduce unsightly bubbles in the main display. This would be a fine start up tank but make sure to have the room to add an extra tank or two into the sump system later VIA bulkheads drilled into the first sump right off the hop(make sure when the 45g is first built you have these drilled and plumbed with bulkheads as you will not want to drill them later) so as to be able to later daisey chain into the setup a refuigium/ extra rock chambers and room for say a reactor or two as we all start with JUST a simple tank in mind but get bored and want bigger and better VERY SOON. This is just my humble opinion nad you must do what is AFFORDABLE and right for you and your needs....... That being said I have a 245g display with a 180g sump and I'm already looking at needing a LOT more sump room as my current foot print is two small for the new calcium reactor and NP reactor i'm getting soon and 3 skimmers Im currently running and still have room for the refugium and extra live rock I do not want in my display tank..... So I most definelty agree with all the other comments
BIGGER IS BETTER
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