20 gallon mass extinction?

[Empyreal]

So I have a 110 gallon and a 50 gallon with cichlids that do very fine and dandy. I have never had any problems, but recently my ego was hurt when my Mom's 20 gallon tank, of which I am the caretaker, had a mass extinction - lost 14 fish. Heres the details:

All the fish were fancy guppies or mollys. They were all relatively new, only 2 weeks at the oldest to the tank. The tank was cycled as it had been up and running for 8 months prior and had fish that were just moved out (no disease deaths). The fish involved in the mass extinction got a white fungus film all other their body before, so I am curious if one of those disease ridden mollies infected the tank. I have an older HOB filter (forgot the brand) and a sponge filter. I tested the water and the only thing out of wack was my nitrates at about 80 ppm. Couldn't be feeding too much because I only gave a pinch a day of flakes.? I also do routine 40-50 percent water changes once a week. Possibly over-stocked?

Three questions:

1) Any idea what I do wrong? - like to learn from my mistakes

2) How to I make sure the tank is clear from disease before I add new fish?

3) Any suggestions for colorful fish for a 20 gallon? I like cichlids, but they had to be colorful ones so my mom is happy.

Thanks guys

 

[Adamrhh]

2) if you do a reading and all levels come back good you should be fine. You could treat with Prazipro and increase the temperature to like 85 to kill any bacteria.
3) rams like Bolivian and German blue

 

[Empyreal]

2) if you do a reading and all levels come back good you should be fine. You could treat with Prazipro and increase the temperature to like 85 to kill any bacteria.
3) rams like Bolivian and German blue

Thank Adam, was thinking of GBRs but I was scared I put them in they would get the same disease. So could I treat with Prazipro and turn the temp up, how long should I do that for before adding more fish? Will it affect my good bacteria treating the tank this way and no having any fish to feed the bacteria ammonia?

 

[Empyreal]

bump

 

[Adamrhh]

Prazipro is a really safe bet, it will not harm your good bacteria. I would treat with the right amount that is for your tank then do a reading and see the levels after a nice water change. After that you should be in the safe. The Prazipro will kill any harmful stuff. Also you could try switching out your substrate maybe it could have gotten something chemical on it that is leaching. I had the same thing happen to my 20 gallon, lost my entire stock to something, I ended up changing the substrate, then deep cleaning the filter then adding old media from my healthy tank so it would still stay cycled then treating with Prazipro, water change right before adding a few fish which survived and were healthy.

 

[Empyreal]

Prazipro is a really safe bet, it will not harm your good bacteria. I would treat with the right amount that is for your tank then do a reading and see the levels after a nice water change. After that you should be in the safe. The Prazipro will kill any harmful stuff. Also you could try switching out your substrate maybe it could have gotten something chemical on it that is leaching. I had the same thing happen to my 20 gallon, lost my entire stock to something, I ended up changing the substrate, then deep cleaning the filter then adding old media from my healthy tank so it would still stay cycled then treating with Prazipro, water change right before adding a few fish which survived and were healthy.

Ok, thanks alot man!