Fish: [tropical fish club] a little OT

I know this is a tropical group, but I hope you'll forgive one excursion
into the goldfish realm.

After our move, I had a 29 gallon tank that was empty. Since I hadn't
kept goldfish since I was a kid, I thought why not. Just for a change.

I got 2 small fantails at PetsMart. One died in 2 days, the other a day
later. I tested the water and it was fine. Yes I used the liquid API
tests, not the paper strips. Not to mention that the same water,
treated the same way, goes in my tropical tank and I've got happy fish
in there. Ammonia and nitrites were zero, nitrates were in the 10-15ppm
range.

So I went back and bought a couple of feeders to use as a test. Same
thing except it was 1 and 2 days this time. Now I'm really worried. I
did a 90% water change, added a teaspoon of ammonia, and waited 24 hours
to retest. Biological filter was still working, no ammonia, no nitrites.

OK, back to the store. This time I took a water sample, not so much for
absolute values since the store uses the paper strips, but to see if my
water tested the same as the water in the store tanks. It did. The
manager of the store didn't think it was my problem, he just thought
thestore was getting bad fish. So based on that I got two calico
ryukins. This time I went up a size on the theory that older fish were
stronger.

Same thing. One of the two is still alive, but I wouldn't take bets on
how long.

Additional info. The tank temperature is 68-70 with no heater. I've
got a HOB filter and a bubbler to ensure oxygen. The only rocks and
plants(artificial) in the tank are ones that came out of other tanks
where they caused no problems. The gravel is aquarium gravel and is too
large for the small fish to ingest, although Iplan on getting rid of it
unless I give up on goldfish. Our water comes from an aquifer with no
added chemicals, but I use AquaSafe+ just to be sure. Supposedly that
eliminates any heavy metal problem although I've never heard of one in
this area.

I'm stumped. I have trouble believing it's all bad fish, but I don't
know what else to tryother than putting in a heater and trying some
platys, tetras, or the like. Anyone here have any suggestions?

Larry B