Most dimensional lumber used for construction is softwood - spruce, pine, doug. fir. Do you know what kind of wood specifically you have?
At 8 pounds per gallon of water, you might want to check the rating on those shelves -- especially for the 37 gallon. Even assuming just half-full of water, that's close to 160 pounds on that one shelf.
I have a couple of those $40 Menards shelving units, too. I don't remember them being rated for anything close to 160 pounds per shelf; I thought they were at least 50 pounds per shelf and at most 100 pounds per shelf. Maybe, I'm thinking of a different set of shelves.
300# per shelf..
evenly distributed of corse.. I have no idea what my floor is rated though!!
Climbing a ladder isn't a problem, i have to do that everytime i clean my 180g anyway.2x4's will be fine
The one question I have to ask is, are you down with climbing on a ladder everytime you have to go into the top tank? You're looking at 72+" for final height. If you've got the space and don't mind needing a ladder, that design looks pretty good.


