Hi Triffid,
Not sure exactly what you're trying to do, so I'm just going to ask questions to hopefully clarify your goals before I'd tackle your more particular questions...
-You want to have a setup where there is a certain level of water for fish? and above that an area where orchids will hang? From your drawing it looks like a cabinet where the midlevel are aquariums, and top shelf for terrariums, but in your writing maybe you're suggesting two aquariums set up where half of either is for fish and top half for orchids?
-When you say top part, not sure if you mean just a cover, or a housing for all the lighting and pipes?
-In regards to the structure, not sure how important it is what the wood and sealant is as long it is reasonable not toxic... reason being if you're working with existing tanks the structure won't be touching anything inside the tanks correct?... there are numerous cabinetry already built specifically for aquariums where the bottom area with open/close doors are for filers, hardware, pipes/etc and just made out of even particle board and a laminate on surface.
-If you're doing an aquarium part with fish you just have to make sure the load bearing frame is strong as water, of course, is super heavy, if water part is not much, you probably don't have to worry.
Not sure if you saw this but I think this is lovely... a post from last year of a setup with water and fish below and vivarium above
Warm orchidarium 750l
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