It looks like that 20g is overstocked and the fish in it are big poopers, so you probably have water chemistry issues. that cloudiness looks like the result of high ammonia, but i could be wrong. does the tank have a smell to it?
do you know the maker of your water testing kit? if you do and can tell us what colors you get in the tubes for ammonia, nitrite and nitrate, that might help track down the problem. I'd recommend treating the tank with prime after at least a 50% water change, followed by 25% water changes daily. The gravel could use vacuumed/cleaned every couple of days - it's amazing how much poop they'll leave around.
what colors are the gills on the fish who've died? i'd hold off on meds until you can determine if water chemistry is the issue. Meds with bad water levels could be more harm than good.
