Hello; This raises a red flag to my way of thinking. Although a 45 gallon is not real big, I would want it to be as level as possible. I have not bothered so much with 5 or 10 galllon tanks, but do level 20 gallon and up.It's 45 gallons. I'm pretty sure the tank is supported by 2x4s. The tank is not level but that's because my floor is not level. I will double check though.
In my experience few floors have been level. Most stands have to be shimmed to some extent due to a slope in a floor.
My take is that a stand is best able to carry a load vertically. When not level, the stresses are directed at some angle off of plumb. If the floor is like many, it will have some slight movement when you walk near it. This can add to the stresses.
I had noticed a slight sloshing in my 55 gallon when walking near it after setting it up. I added some modest bracing to the floor joists in the basement which stopped it.
Good luck