I'm never a fan of overbuilding tank stands, however in this case I'm a bit concerned for your setup for a couple reasons...
1 - Deflection. Just because it will hold 350# on each shelf, does not mean that the shelf won't bend a fairly substantial amount under that sort of a load. The fact that it appears that your tanks overhand the shelves a wee bit in the front and back also give me the heebie jeebies. I'd like to at minimum see a bit more beef to the shelves, either in the form of about 1.5-2" of plywood glued screwed and fastened into a single slab between the shelf and tank. Allow me to explain my cause for concern... for a time I had a set of plastic shelves that were "rated" at 200 lbs per shelf. I had a single 10 gallon tank upon one. It was ok for a time, but over the span of months the 80 lb tank gradually deflected until i could slide a pencil between the tank and the shelf. the tank itself was holding up ok, but again, it was only a $6 tank and a few gallons of water. no huge risk. you have the makings of a very nice looking setup that you have already made some substantial investment in. I'd set that shelving unit on a concrete floor, empty, and measure the distance from the floor to the shelf on each end and in the middle. then put a couple 3 or 4 gallon buckets full of water, on it, then sit on the buckets and have a buddy remeasure from the shelf. I'm hoping you will see a difference of less than a 1/16" or so, but I wouldn't be surprised at all if the number wasn't closer to 3/8" or 1/2".
2 - point loading. Are those 4 spindly little legs sitting on concrete or some other flooring when this thing will be in it's final home? If the floor is anything but concrete, I'd recommend putting something under the leg to spread that load a bit. I'd used a 3 or 4" square of 1/4" thick aluminum or steel under each leg. I would not use wood, MDF, any sort of plastic, or anything that could absorb water. I also wouldn't recommend a patio block or any "concrete product" less than 3" thick.
I'll get off my soapbox now, but I hope you at least take what I said at least under advisement. At the very least, please load up the stand with ~300 lbs per shelf and shake wiggle and measure some stuff before you start gluing pipes together.
Good Luck!!!
