Flow Rates and Small, Young Fish

[Marlin65]

Thats possible as well. Never thought about that.
How big is your return pump and your slots on the overflow?
Your in tank flow should not mater so hook that K3 back up.
I had a small file fish and one of my clowns get wedged up against the overflow and get killed, but I run a 3500gph Dart on my tank as my return.

[reefbyremote]

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.

[fkshiu]

Keeping the skimmer on at night and increasing surface agitation with powerheads is all you need along with removing any glass covers.

An airstone will create a lot of salt creep. Lowering SG to 1.021 will be hard on your inverts in the long run.

[Kevotron]

If it is an oxygen issue, why not just remove the tank lid all together, and use the same plastic grid to cover the top so no fish will jump out ?

[reefbyremote]

I hoped to keep the lid on in order to minimize evaporation as for now I do not have an auto top up. As a "middle ground" I am considering using the same plastic grid to replace glass feeding sections at the front of the lid. The plastic grid we used is too thin/flimsy to cover the entire top of the tank without a frame, so egg carton (or is it crate?) would be better for that purpose. Thank you for the help Kevotron.

In the meantime I have propped open the lid's front sections about 1/4 to 1/2 inch. With this and the other changes (skimmer running 24/7, refuge light on, k3 pointed at surface, temp down from 79 to 75) the fish have been active and acting normally now including eating for 1.5 days. Oxygen is clearly no longer a problem (but at least I know now very well the early and later symptoms.) I plan to monitor a few days before making further small(er) adjustments again. fkshiu I will listen to your warnings about reducing salinity and the use of a bubbler with an airstone and not make these adjustments either. Thank you.