Hi SS
some advise
if you can get together with Caryl, this will help
start small with everything but the tank, you had two many fish in the tank for its size, especially during cycling, I know this does not help now but do not buy anymore fish untill things stabilise.
cycling this is important, go slowly, very few fish and frequent smaller water changes are better than infrequent large changes, treat the water to remove chlorine (in your case get a new bucket that is only for the fish wash with water only, no soaps, fill with 5 - 7 litres treat and leave till up to ambient temp, change daily)
Ph test kit is important, also ammonia, nitrates, nitrites, if you are close to a pet store they may be able to test a sample for you, if not then again Caryl can help.
Medicating the tank may be necessary now, but be aware carbon in the filtration system may need to be removed as it can suck out the meds. please tell us what meds you have used.
Lastly water quality is the most important thing for your fish and in such a small tank things will go wrong rapidly if it is not right.
sorry for your lost fish, and Caryl sorry for making free with your name
Mark