I'm establishing a new pond. It has a small stream about forty feet
long, river rock mortared in, with gravel bottom, and four small (a
few inch each) falls. Right now, it's not so much a pond as a fifty
gallon tub at the bottom of the stream, until I get around to digging
the actual pond. I'm running several hundred gallons per hour through
the stream (it's primarily for a pleasant sound effect outside the
bedroom window for now), so I'm turning the system over several times
per hour.

The cement and mortar are several months old now, and I've changed the
water a couple times, so I suspect that the lime is mostly leached out
by now. I know there's a good biofilter situation going on
(presumable in the stream bed), because for the last week, I've put
pure ammonia into it, spiking it up to several parts per million, and
it's back down to zero within a day or so, with a corresponding
increase and decrease in nitrites. The ph is a little high (near 8)
but when I add acid to take it down, it goes back up again within a
day or so.

Anyway, I decided to try some feeders in it. I put in eight
yesterday, and as of now, three of them were floating. I don't see
the others (but it's a shaded area, with a fall into it creating
turbulence, so I wouldn't necessarily). Anyway, three out of eight
dying in one day sounds like more than normal mortality to me.

What should I be looking for? Is it possible that it's just too high
a turnover rate, and the pump suction in such a small volume is wiping
them out?

The problem with that theory is that before I cemented in the rocks,
and was just running it with a liner, I kept fish in the tub with no
problems (though they did die from other causes (e.g., an overflow
when I wasn't home that flooded the yard and then stranded them).

Or perhaps I didn't give them long enough to stabilize temperature
before releasing them from the bag, and they're shocked (I gave it
over an hour)?

Anything else I should check?

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