[billdogg]

Even with my system running at at least 80x the fish not only aren't bothered by it, they seem to enjoy it. My Tomini Tang likes to "surf" in the outflow from the gyres, and at feeding time, with that much rather chaotic flow, the food is all over the place, much like it would be if it were live. It seems to stimulate everyones hunt it down and eat it response.

As to your question about zero head pressure - that is the maximum output from a given pump when the inflow and outflow are at the same level - think powerhead in a bucket if you will. Head pressure is the height of the water column that the pump must push against. Assuming a "normal" setup with sump below the tank in the stand, you are looking at about 4' that the pump must push against to return the water to the tank. All pumps have a maximum head pressure after which the pump will continue to turn but no water will come out. For the pumps we use, this pressure does not hurt the pump at all, but must be taken into account when thinking about total flow. Many pumps have a fairly dramatic dropoff due to this - hence the need for "flow" pumps vs pressure rated pumps. This is especially true for those whose sumps are located in the basement underneath the tank.

hth!