PARALYZED GUPPIES AND BETTA

Question
I have two tanks and l got fin rot in one of my tanks (3weeks ago) which l treated and my fish have recovered very well.
But now l have had problems in the same tank with fish becoming half paralyzed and in the end dying (I have lost so 4 so far 3guppies and 1 mature betta) They start showing symptoms a couple of days before and not eating and there colour becomes very dime as soon as they become paralyzed they die with in two hours.
I have checked the water and all tests have come back fine. The tank has been up and running for 2 years with out any problems. I did add fish just before l got the fin rot (I belive this is where it has come from) I have not treated the water ( as l don't know what is wrong I've just just done my normal water change. My other tank has not had any fish added as l have had no health problems l do the water changes at the same time and the water checks have come back the same as the other tank. Any idea on whats wrong with my fish?

Answer
Sounds like toxins in the water, do a 100% water change to clear out the toxins and look into installing a carbon filter. Next time your fish show signs of becoming lethargic put in some sea salt to see if it will revitalize them. Only a tablespoon per 5gallons. Hope you have better luck!