If my memory serves me correctly, Tidal Gardens used to have a lot of their tanks supported just on the ends with cinder blocks with nothing spanning between them. It’s important as you said that anything in the middle is even or slightly lower to prevent a high point.
With standard rimmed/framed tanks, like the OP has, you can support them from either the ends or across the two long sides. The frame adds a lot of strength.
Clearly, you wouldn't want a "hump" in the middle of your frame's rail or crossmember, you do want to take some care and make sure all the vertical support pieces including any center supports are the same length, whether you cut your own or have Home Depot do it.
As said, you wouldn't want a hump in the center, but I don't really see that as a big concern here. The OP's tank is just 48" long, I don't see how they'd manage to fit a center support piece in place that would be so long that it would push the rail up in the middle.
To the OP, dry fit your pieces before assembling, meaning, you can drill and screw the frame pieces together to see if everything is coming together as you planned. If things are lining up and looking good, just back the screws out and reassemble everything with your wood glue.
And, something to keep in mind is that it's very important to make sure that all the vertical support pieces are cut to the same exact length, but it does not matter what length that is. What I mean is, if your stand plans called for the vertical pieces to be 30" long, for example, and you cut them to 29.75" or 30.125" it doesn't really matter as long as all the pieces are the same length. The only thing this will change is that your final stand will be a fraction of an inch higher or lower than you planned on paper. Not a big deal for most of us, unless you require an exact height.
My Home Depot has a good panel saw and a radial arm saw, and the radial arm saw has a stop block. Tell the person helping you that you really need at least the vertical pieces to be all the same. I would suggest you ask that they use the radial arm saw with a stop block, they should know how to set the stop block to whatever length you want, and then all the pieces should come out exactly the same length. It can also be done on the panel saw, so if the person is confident, I'd say go for it. It's only a 2x4, they have more of them ;-) Either way, have them slice one or two and then stop and check with your tape measure.
Whether or not Home Depot does a good job cutting your material really comes down to the person who happens to be helping you at Home Depot. I've had pretty good luck with them for the most part, so maybe give them a shot if you can't find your own saw to use. I mean, we are talking about a few bucks worth of 2x4's to try it, and if the Home Depot person really screws up, you don't have to buy the wood, they through it on their discount pile. Bring your own tape measure and double check what they are doing.