[melev]

The water would be pumped up from a common return area.

Actually, the corals would benefit from the fish as their food and poop can be used by the corals for growth. As long as your reef stays within healthy parameters, the frag tank should do well.

Then again, if you put pests in your frag tank, they can migrate to the reef, so there is that risk. In that case, you'll want independent systems but keeping both stable can be challenging. Combining the water volume tends to be quite stabilizing. You just need to quarantine new arrivals including any corals or frags to avoid introducing pests into your reef.