Personally, I think no. I had a 40 breeder and a 20 High on a 2 x 4 shelf and it caused the 40 to leak due to the 2 x 4 bowing. although, the 20 highs individually will be more rigid than the 40 breeder due to its dimensions, however 6 maybe stretching the limits 4 likely ok, 5 maybe, 6... I personally wouldnt do it.