fish keep dying please help.

[cheesy318]

About a month ago my hob filter quit working and I was out a filter for 3 days. I lost a baloon Molly. I did daily wc and fed less. Did daily parameter testing, everything was normal When I got the new filter( cf300 aquatop canister) we downsized tanks. From a 29 gallon to a 20 gallon. At the time of the down size, tank was stocked with 2 powder blue gourami (1male 1. Female) 2 dalmation mollies and one peppermint Platy. I have lost both gourami and just today I lost a dalmation. I have examied each corspe closly, they have not looked bloated, poofy, or white spots. Their gills have not looked red. My parameters are
Ph7.4
Ammonia 0ppm
Nitrite 0ppm
Nitrate 5ppm
I just did a 2.5 gallon wc last night. Temperature of the tank is 78f
Should I treat broad specturm? If so can I treat melafix and primafix at the same time? Or should I treat something else?
@
Should I be testing anything else?

 

[rmiller]

Did you notice any symptoms or strange behavior previous to the death?

 

[cheesy318]

Nope nothing bizzarr.

 

[rmiller]

So now all you have is one molly?

 

[cheesy318]

A Platy and one dalmation