1/4" cast acrylic will suffice.... as for a rule of thumb it's pretty much just exp. that tells you..... it's not about heigth but more of length x heigth...thanks for the info. how thick should a 14x8x7 tank be to be rimless(thinking about making a desk top frag tank or micro reef) , also whats the rule of thumb on deciding how thick the acrylic should be?