You can use 2"x4" on all 4 sides of the tanks to keep it from moving then a couple of large ratchet straps over the top with large eye bolts through the floor. I use large washers or large washers with a board kind of like Pat did to help secure the bolt from easily ripping through the boards.
One tank should be good unless you are hauling water, I would make sure that with the tanks full your trailer is not more than 80% of it's capacity. 8.33 pounds per gallon x 550 gallons of water = 4581 pounds. If you have a typical tandem axle trailer with 3500 pound axles you are looking at about 1100 to 1500 pounds empty trailer weight then add the water on top of that and you are at 5681 to 6081 pounds, depending on how heavy the empty trailer is. Then add your equipment, chemical tanks, hoses, reels, etc.....
If you are running 5000 pound axles you still have room to grow, if not then I would be careful with how much weight you add.
I had too much weight on my old trailer and hitting a pothole with only 200 gallons of water in the tank bent an axle, each time I bent an axle it cost me about $600 between new tires, axle, labor to install the axle, welding brackets on the axle, etc... The tires would bald in about a week or two then have to replace them after flipping them over on the wheels to get more life out of them until the trailer place had time to change the axle.
Now I don't have that much weight on the trailer anymore and try not to haul a full or near full tank of water anymore.
Be careful.