The fish survived the night and some inadvertent changes I made gave me clues to the problem. The short story is this appears to be a low dissolved oxygen issue. As a newbie with a lot of fw experience this was a great learning experience. It appears I made quite a few mistakes all at once. At the risk of looking like a fool I decided to post the details.
In fact besides the powerhead, I then turned off my skimmer last night and the sump light in an effort to minimize the fish being spooked. The result within 1 hour in the dark was the clowns struggling at the surface in the return water area near the overflow, and remarkably the 3-4 inch goby- who had never left the bottom before- stuck against the overflow. I pushed the fish all away from the overflow and they were all very groggy/sleepy- probably on the edge of death.
After scratching my head I realized it was an oxygen issue (skimmer was never running when it happened) and re-started the skimmer, sump light and turned on and adjusted the powerhead to shoot much more directly at the surface to create turbulence. I also propped open the tank lid (I was running covered to reduce evaporation) and turned down the temp from 79 to 74 (lower temp = higher potential dissolved oxygen.) Within 15 minutes all was much better. Of course I woke 2 hours later at 2 am to check again and the fish were acting normally- the clowns in a corner out of the flow and the goby in his cave.
FYI the return pump is rated at 650 gph. the overflow slots appear to be the standard size about 1/2 inch or 1 cm. (The tank is a standard store bought.)
Today we have put some finer plastic grid over the overflow grate just in case. We got it from a craft store and cut it to size at about 1/2 cm and it does not impede the over flow.
So far so good, the clowns seem to have called a patch of brown mushrooms in the centre of the tank 8 inches from the bottom "home" and are eating and picking at the zooplankton/pods in the tank. The goby is continually sifting. Sand flying everywhere...
To increase our safety margin we are also considering a) using a small airstone with a low flow of bubbles in the return area of the sump or the tank it self and b) decreasing the salinity from 1.25 to 1.21 to have some safety margin (lower salinity at same temp = higher potential dissolved oxygen.)
I would apprecaite any feedbacks others may have. Thank you.
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