DON'T DO IT!!!
I have a lot of experience with those shelves and they do not hold up well as they age. I believe they are only rated for about 800-1000lbs total, not per shelf...at least none of the units I've used have ever come close to holding that much and have broken with a lot less. And yes, I have broken those types of shelves before with much less weight than you are planning to put on it.
Do yourself a favor and take the time to build or buy a stand sturdy enough handle the tanks. I'm telling you, if you use that unit, whether you modify it or not, it will eventually fail on you and you will lose everything.
View attachment 1187425
Here is a picture of one I broke. That white pump that is in the bottom of the picture weighs about 75lbs and was the only significant weight item on the shelf when it broke. You are wanting to put about 20x that weight on it...it will not end well!!!
I'm not trying to be an a-hole, I just really don't want to see anybody lose any of their equipment and when I have experience with something first-hand, that I know will not work, I am very adamant about stressing the importance of staying safe and not going forward with it.
Please, please, please, take my advice seriously, do some research and either build or buy a stand that will handle the weight of those tanks. You are looking at over 1500lbs easily, possibly closer to 2000lbs. It's not something to take chances with.