closed-system rewatering and plant wastes?

[dethswatch]

I've got a wardian case setup that mists and waters and then drains into the same container. The water is exhausted every week so it gets new (tap) water at that point.

I think Ray mentioned that this wasn't great due to plant wastes, etc.

What sort of wastes are we talking about and how would I go about analyzing the water to determine to what extent I've got which wastes?

Can anyone tell me about plant wastes or point me in the right direction so I can read some more?


Thanks