I have a question regarding whether it is okay to allow the drainage water from under the substrate to mix with the water feature?

I will be building my first vivarium from a 40 gallon breeder tank. I plan on having a water feature consisting of a small pond with a water fall and a separate stream flowing into that pond. I will be elevating the substrate above the tank bottom with plastic egg crate sitting on some pvc spacers too allow for drainage of the substrate. Do I need to keep the water feature separate from the drainage water?

To circulate the water I will be using a small pond pump that flows 55 to 80 gph to pump threw a filter then over the waterfall and also to the other side of the tank to supply the stream that will flow back down to the pond. I will construct the filter from scratch and hopefully include a bio wheel with the filter media.

If I can mix the substrate water with the water feature then one benefit of this is I can use the same pump with additional plumbing to pump out excess water from the tank as needed.

Comments would be appreciated!

Mike