I am really sorry your fish passed away, and another is sick.

I would QT your sick fish right away, and put a filter, if you have an extra, in the bucket with it (rubbermades are good for this). I wouldn't worry so much about keeping the water shallow, as it may very likely not be a SB issue. As far as I am aware (and I am far from an expert, tiger Barbs aren't too prone to SB issues, but correct me if I am wrong) and my expereince with SB is that they almost always float not sink because of it. I had a GF that had SB for over a year with no ill effects.
Can you tell us more about what the fish looks like and its behaviour. Remember that it is best to QT any new fish, as many you buy are already infected when you buy them, and that way avoid infecting your healthy fish. Maybe try doing more frequent water changes (you should be doing them weekly, but not necessarily doing a total cleaning every week), even do one now. If nothing is affecting your other fish you already had, I would guess the fish was sick when you bought it. Maybe try to contact the LFS you bought them at, and maybe they have had problems with theirs in the store since you bought yours and can give you info., as well as the fact that some LFS will replace the fish if they die. Keep in mind though, many LFS have no real knowledge of how to care for or treat ill fish, so research anything before you do it. Did you also check what the stors water paremeters were? If they were very different than your tank, the fish could be going into shock. It is very important to ask them what tehy keep the temperature, pH and such at so you can aclimatize the new fish to your tank without harm. Sorry I am babbling, but I am tired and my mind is racing all over the palce with ideas (I have the most experience with GF illnesses, not tropicals), but I din't want to leave you hanging wondering what could be going on.
Maybe if you could post (I have taken this from the GF disease section, but the info. can still really help to diagnose any fish) Test Results for the Following: (It is hard to guess what might be wrong without more info. as there are many reasons a fish could be laying on the bottom and breathing hard).
Ammonia Level?
Nitrite Level?
Nitrate level?
Ph Level, (If possible,KH and GH and chloramines)?
Ph Level out of the Tap?
Tank size (How many Gals) and How long has it been running?
What is the name and size of the filter/s?
How often do you change the water and how much?
How many fish in the tank and their size?
What kind of water additives or conditioners?
Any medications added to the tank?
Add any new fish to the tank?
What do you feed your fish?
Any unusual findings on the fish such as "grains of salt", bloody streaks, frayed fins or fungus?
Any unusual behavior like staying at the bottom, not eating, ect..?
I'll check back tomorrow and see how things are going. Good luck and I hope your fish are doing OK.