My fish are getting smaller and then dying...Help me please D:?

It could be that you are simply underfeeding them. Or, if you had too much fish for your tank, your fish could have suffocated. General rule of thumb - 1 inch of fish per gallon of water. (5 gallon tank = 5 inches of fish) Obviously, you can raise the bar by adding ornaments that pump bubbles into the water and thus oxygenate the water.