ED wrote,
How much gravel / load could one expect to safely
place inside tanks of this construction.....

I have sloped the gravel up to about 6" deep, then piled 8 to 10"s of
slate atop that without any problems... I have a friend that has 4" of
gravel in his larger tanks (two 70 gal. tanks) for nitrAte removal, a
fresh water deep sand bed. Doesn't look so great, but it does help to
keep his nitrAtes down............ Frank