to properly cycle a tank, you want to create ammonia in the tank which will then be broken down into nitrites and then nitrates. easiest way to remember the order is look at the end of the spelling of ammonIA, nitrItes and then nitrAtes. ammonia is produced by waste and excess food in the tank. you can either do a fishless cycle or cycle ith fish. cycling with fish is very stressful on the fish and you can lose fish. to cycle without fish you need to create ammonia by either feeding the tank with food and letting it decompose in the tank or adding ammonia to the water. if you add the ammonia it has to be pure ammonia. if you shake the bottle and it bubbles it is not pure. get a test kit and test for ammonia in the tap water, should be zero and after every drop of ammonia retest the water. when the level reaches about.5 stop adding the ammonia. once again you cannot do this with fish in the water or plants. wait about three days and do a water change of about 10%. then on a weekly basis do subsequent water changes of about 10%. you will notice that the ammonia will drop to zero, or zero out, and then the nitrites will rise. this is good. you are establishing your bacteria in the tank. nitrifying bacteria is good. it "feeds" on the nitrites lowering and removing them from the tank. if you add too many fish, once your tank is established, at one time, the nitrifying bacteria is overloaded with the large waste load and cannot reproduce to handle the load. so you will want to add fish slowly to et your bacteria be able to catch up. once the nitrites are lowered, the nitrates will rise. nitrates is the lowest of the toxins but still harmful to your fish. plants will help to remove nitrates from the water. aquatic plants feed on nitrates just the same that house plants feed on carbon dioxide. once the tank is established or cycled you may notice a slight fogginess to the water. that is referred to as the tank has bloomed. the biological filter is established. your biological filter, along with your mechanical filter will help to rid the tank of unwanted toxins. everything that is in your tank will have the bacteria in it. when you rinse your filter you will want to rinse it in tank water to keep the bacvteria intact in the filter. you will want to vaccuum the gravel on top so as not to disturb the bacteria that is in the gravel bed. after the tank has been fully cycled, you can start to slowly add fish. 1 or 2 at a time. to fully cycle your tank could take up to 6 weeks with a fishless cycle. jumpstarting it by adding the ammonia will cut the time in half.
you are experiencing fish loss cause your tank is not cycled. purchase a good test kit that will test for ph, ammonia, nitrites and nitrates and test your water. see what stage you are at. more than likely you have high ammonia in the tank and some nitrites. do a water change of about 10 - 20%. smaller tanks are less forgiving thatn larger tanks. you will have to do regular water changes imo atleast twice a week when it is cycled. if you are wanting to keep fish, it is better to start off with a larger tank. 5 and 10 gallons are only good imo for hospital tanks, fry tanks and grow out tanks.
hth
maggie
So I quickly changed the water, but when I moved the fish out from the tank, I noticed that one of the platies died too!! And now, the other molly just stayed at the bottom of the tank, he used to be so active & lively. 
Can anybody pls tell me what I should do? I'm totally new when it comes to this (my 1st time having fish) so any advice is appreciated! Thanks!!
I have a beta dieing I realy love it I got it a year ago. All it does now is sits in the bottem of the tank under rocks and a statue of miss piggy it hurts to see it suffer if only I could help it.But im cool 
