Two things, first the 1000lbs is the total weight limit. Only 200lbs per shelf. One 20g is easily going to be 200lbs. I do not like to keep a shelf loaded to it's total capacity permanently, long term. My guess is that this material will sag or deflect somewhat.
Another point, this looks like it snaps together. That would mean that you could not adjust the spacing of the shelves. You may not have enough clearance to get your arm into the 20g for routine maintenance.