Thanks to all for the responses. I was a little concerned about
changing the bio-bag, but it had been running for about 3 months and
had noticed a decrease in the flow of water back into the tank (along
with the overflow). You mentioned that the plastic frame from the
bio-bag is designed to retain bacteria. When I replaced the bio-bag,
should I have removed the frame from the old bag and put it in the new
one, or just changed the entire bio-bag (like I did)? Again, I'm not
sure if that was the cause or not.

Doing all three in one day was just us being impatient. Of course,
we're learning more and more how patience is one of the keys!

I'll continue monitoring my levels to see if the Ammonia keeps rising.
The first one that died yesterday was probably dead in the tank for at
least half of the day when I was at work. The second for not very
long.

Jon