A Mystery As To Why My Fish Keep Dying?!?

"This isn't your ordinary question about why someone's fish are dying, and then its revealed that the asker actually keeps 5 Goldfish in a bowl and uses no chemicals other than tap water." That's kind of what you're doing. Keeping aggressive fish in the same tiny tank. Paradise fish need 38 liters a heater and gentle filter. You cannot keep males together and you can't keep males and females in the same tank unless the tank is much later say around 75 liters at the smallest. You were keeping two fish in a 15 liter tank... that was too small for them. Even that five gallon tank with 4 paradise fish in it was grossly overstocked. The only thing that can live in a five gallon tank is a Betta and only singly. Even with the semi-okay care you've been giving them you were bound to have issues for keeping that many in a tiny tank. Discard all but the 5 gallon tank and get a Betta.