Do it.
I did it for my fishroom, three sets of stands, variable heights for different functions (overflow tank, main sump, frag) topped with a 2x4 frame the same dimensions of the particular tank and a 3/4” plywood sheet
Some say it looks crap- I like it. Not showroom quality, but looks fine in the room.
Leveling is done with composite shims, places between the 2x4 frame and the blocks. Start off with a level floor and use the same brand concrete (not cinder- they haven’t been used in decades) blocks - some brands differ 1/8”, and it’ll need minimal shimming
I didn’t bother painting the blocks, but drylok will do it. Water won’t damage the blocks, and, actually, if you do get a slow leak somewhere, you might notice the discoloration in the block as a clue.
Don’t use foam board if the tanks have frames- not needed.
Good luck and post pics.
More recent pic of the freezer under the 48x24 “ frag tank (50g low boy) stand. Had to elevate with an additional couple pieces of 2x4 as spacers
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My build thread:
https://www.reef2reef.com/threads/neil’s-in-wall-180-with-basement-fishroom.390436/