"The Ranger" wrote

I'd like to build a stand for three 10-gallon fish aquariums for my
classroom. Preferably, I'd like something that is strong enough to handle
the weight from the water, have an area underneath for storage, and
something that cannot be pulled over should a young child "lean" on it.

You want a table with a lower shelf. Can you bolt it to the wall? If so,
you have the stability needed. A simple 90 degree angle bracket to the base
and wall will do this.

If you want 30G water total on one table with 3 tanks, you will need a
center pole at front and back or the wood will bend, based on standard sizes
of tanks.

1 inch thick plywood for the top, cut 14-24 inches 'deep' depending on your
wants for display area before the tanks. (cut to 24 inches if you have the
space and set to 36-40inch tall and you do not need to bolt to the wall). A
standard 8ft long will work for this or you can tighten for most tanks to
6ft.

6 posts of 2x4 (can get away with 1x2 for center support set but creates
problem with cross-pieces). May want to split this every 2 feet if you
want 8ft long top so need 2 sets of center posts.
Corner-center-center-Corner.

Now the harder part. I personally would use 90 degree metal angle brackets
to secure the lower shelf and legs to each other and prevent 'wobble' but I
bet you want to cut a top stabilizing set that fits between each 'leg' at
the top and perhaps sister the legs at the bottom for the bottom shelf. You
need to be sure the kids can't pull it over.

To secure it, look at your standard coffee table. See how it has a wood
part cut to fit between the legs? It's not there just for looks. You will
pretty much build the top then then legs then flip it on the backside (legs
up) and measure carefully and cut to fit pieces that fit between along the
legs and nail or angle bracket them in. Flip it back to standing position
and nail a shorter bit of wood to the legs that starts from bottom up to how
high you want the second shelf underneath to be, then cut the plywood so it
leaves room for the legs and just rests on them.

I hope this helps.