Quite a few people have their sump, water change tanks, RO/DI in other rooms. So that is not that unusual. Now I only have FW tank right now but would like to toss some things out there.
If in the garage, there are things to consider:
1.
Is garage just a storage place? If so, what are you storing in there? If you're storing chemicals, lawn fertilizer, gas for the mower, etc. I'd be concerned about slight amounts of these floating in the air and getting into your sump then being transferred to your tank.
When you go to clean the outside of your tank, I've always been told you don't spray the cleaner on the glass or even in the same room. You spray cleaner on your cloth in another room & then clean the outside of the tank.
So if you have chemicals or fertilizer and are pouring it in containers, powder will be in the air. Similar to cleaning a litter box.
I remember someone did a study one time & said when we flush our toilets, small droplets of water we really can't see can be disbursed about 8 ft. away. Think about that the next time you go to brush your teeth.
2.
Or is it actually used? If so, when you pull your vehicle - i.e. car, lawn mower, etc. - into the garage, you're bringing in exhaust fumes from the vehicle and they are in the air. Again, chance for them to get into your sump and transfer to your tank.
Now, maybe I'm totally wrong here but just some things that come to mind.
Now one of the advantages to having your sump away from your tank is the reduction of noise from the pumps running to push water back into the tank. Plus if the sump isn't under your tank, you possibly have a little extra room when you're working on it.