Advice needed for new aquarium with potting soil substrate and dying fish

I originally posted about my problem on r.a.f.m. but after doing several
hours of research this morning I now think it might be related to my potting
soil substrate so I'm posting this here. A really short summary: I set up
a new 60L (15 gallon) planted aquarium with potting soil substrate (about
1") and quart on top (about 1"). I added a couple amano shrimp and they did
fine for a week and then both of them starting dying at exactly the same
time. They started lying on their backs and swimming very eradically, and
they turned white. I immediately took them out and put them in my
neighbor's tank, who has been helping me set up my tank like his (using
potting soil substrate). The shrimps recovered in his tank and I left my
tank fishless for one month. I then added an ancistrus. The next day, just
before going to bed I checked on him. He had been doing fine for over 24
hours (I checked at least once an hour) but when I checked he was on his
back. I touched him with the net and he swam eradically (just like the
shrimp). Twice he swam up to the top and actually poked his head out of the
water, and then swam more and went on his back. I took him out, intending
to transfer him to my friend's tank like I had done to save the shrimps but
he was already dead.

I am now suspecting the potting soil. My water is a brownish color, which I
originally thought was algae but now I think it's due to the potting soil or
both. I read that the potting soil can contain fertilizers and other things
that can consume oxygen and/or give off marsh gases. The tank does smell,
but it smells more like potting soil than rotten eggs (sulfur). When the
ancistrus died my wife said she thought maybe there wasn't enough oxygen
since he poked his head out of the water. I didn't think much about it
until I read today that potting soil can consume oxygen. I'm really at a
loss as to what is wrong with my tank and what to do about it. It was
recommended to me in r.a.f.m.that I should tear down the tank, clean it with
bleach, and start over. It was also recommended not to use potting soil. I
know some people do well with potting soil, such as my neighbor who has a
great tank for over a year now, and others hate it. But would potting soil
cause the behavior that I described? The day the ancistrus died I added
back one of the shrimps that I had placed in my neighbor's tank and while
the ancistrus was dying the shrimp was doing just fine! I put the shrimp in
my neighbor's tank because I didn't want to take a chance on him dying.

I find the whole thing really bizarre and if you have any advice please let
me know. There are lots of choices for substrate and there is no clear
obvious choice, so this is a tough decision for me as well when I rebuild
the tank. Many of the suggestions I see such as vermiculite and flourite
don't seem to be available here in France.

Thanks,
Michael