from experience, as long as the stand is varnished well, humidity will not be an issue.
just because i always take the precautionary route...(well u never know!) i am not overly confident with the base. from the pictures, all of the uprights are are on top of the flat base with only 4 struts at antagonist angles. i have no doubt it could support many many kilos with only down force its just the swaying behaviour of water.
ie eggs are super strong top to bottom but side to side,,,, not so.
my own personal preference is a rectangular frame with inside frame struts. the top frame is then a mirror image of the base. the supporting struts then sit between the upper and lower frames at antagonist angles. cross beams and uprights are then added at 1.5 feet intervals (over 3ft tanks only) each with parallel and antagonist angles (to beams at a right angle) depending on the hight of the tank i would add some arch supports as you have.
any way good luck and good work