(please do not type in capitals as it indicates you are shouting)
Can you post us your exact ammonia, nitrate and nitrite stats? Fry are very sensitive to these things and if your tank is fairly new i would suspect somthing is amiss in it.
A 5inch by 5inch tank is incredibly small, more a bowl, and is too small to put a filter and heater in it which the fry will need. A 10gallon tank is far better for raising swordtail fry as swordtails can take a year to mature, anything under a 10gallon tank will only be able to raise 1 or 2 fry to full grown at best.
You need to know about cycling tanks and how the ecosystem in fish tanks work to be a successful fish keeper and breeder. Aquarium fish are essentially living in their own toilet and fish poop creates ammonia which in turn is lethal to them in any small quantity.
This is where the tank filter comes in, its main purpose is not to only make the tank water oxygen rich but to grow beneficial bacteria in its sponge inside it capable of breaking the ammonia and nitrites(nitrites are also lethal to fish and build up when there is nothing to process them) down into harmless substances, but when you buy a filter for your tank there is none of this bacteria capable of keeping your tank healthy in it so what do you do?
We do a process called cycling, you can do it with fish and also without fish. The good bacteria needs an ammonia source to break down so it can survive and fish create ammonia but ammonia is lethal to the fish themselves so to help create the right balance you need to understock your tank and do small water changes once every day or so.
This where your second issue comes in, tap water contains chlorine which kills bacteria and simply dumping in fresh tap water into your tap or washing the filter out in it kills off all your hard earned filter bacteria. You need to buy dechlorinator(somtimes called water conditioner) from your lfs(local fish store) and add to your tap water before you do anything to it and the tank related things to remove the chlorine from the water, chlorine not only kills filter bacteria but it can burn fish's skin over time, lower their life expectancys and make them stressed and ill- so dechlorinator is essential for having a healthy tank and fish.
You can also do a thing called fishless cycling as an alternative to cycling with fish where you simply buy pure ammonia and add it to the tank over a period of time before you stock it with fish, the benefits of fishless cycling is that you do not put any fish's health at risk and it takes alot quicker to do than cycling with fish.
The other alternative to getting the filter bacteria you need is to buy a chunk of mature filter sponge from your lfs and place it in your own filter sponge, this is called cloning.
However you go about cycling your tank you will need to test it for ammonia, nitrates and nitrites at least once every couple of days during the cycling period of time to help you know exactly at what stage your tank is cycling at or wether there are any water quality issues.
Fry of any sort of very sensitive to water quality conditions and can die off at the slightest sign of ammonia or nitrites and this is likely what happened in your scenario.
ps: for more info on cycling tanks check up the topics in the pinned articles in the beginners section
