2 Story tanks

[ThePBM]

The to[p tank would have to be sealed airtight, and the inlet aon outlet intot he bottom tank would have to be below waterlevel at all times.. Then the top tank couldn't drain into the bottom tank, any water to fall from the top tank would have to be replaced by water drawn up from the bottom tank since the top is airtight... would create a vacuum effect.

that's absurd.
i'm not even sure what you imagine the OP doing, but he isn't drilling the bottom of his tank or anything like that. a standard overflow that is maybe 2 inches from the top of the aquarium would safely run a safe amount of water down to the lower tank. it would overflow at whatever rate the pump pumps at.


my suggestion though, like some others have said, is to put the fish in the top tank. the bottom tank will be fine with just plants and that's where you'll be putting your pump anyway. throw a prefilter on the pump and the tank with fish will be getting clean water instead of having to live in water that has not yet just passed through a filter. sponges and most filter media will have more surface area than plants so it's best to run it that way (mechanical then chemical) anyway.

 

[excuzzzeme]

Our LFS uses a 3 tier system with one pump. No aquariums are sealed. All 3 have an overflow or return line and filtered water is pumped from a sump filter into all 3 at once. Some tanks are planted and some are fish and all 3 use the same sump. It is very effective and requires little maintenance. The sump is a trickle filter setup with filter media and bio balls in a divided 20H. They use 1 sump for each 6 tanks. Because all tanks are drilled the same, they maintain the same water level. Fish tanks are above planted tanks. Each tank has its own valve to adjust water flow into tank.