Ich

[kpreece]

So my three tiger barbs had been doing fine the first week in the tank so I decided to add a few tangerine mollies and a pleco. The next day I noticed white spots on my tiger barbs; I quickly did a 40% water change with API stress coat. I have began raising the temp of the tank and will add some aquarium salt. The fish are still active and swimming around the tank. Anything else I should do??? The mollies and pleco are not showing any white spots

 

[imamesshelpme]

Did you add water from the tank the mollies and pleco were from into your current tank? If so, the water probably contained something that the mollies/pleco were able to overcome but the barbs were not. Doesn't sound serious, so just do frequent water changes like you said until the situation improves.
Also please do some research on whether your pleco can handle AQ salt. I know that bristlenose plecos aren't able to; always better to be 100% before adding something to the tank.

 

[kpreece]

You know that could be it. One barb died in the night so I bought some ich treatment before work and put it in the tank. When I got home from work another barb had died. The third barb looked better but the mollies kept nipping at it. I went to get a couple more barbs for protection..lol... but when I got back it had died... I'm truly sad...

 

[imamesshelpme]

I'm so sorry.. I haven't heard of ich affecting and killing fish this quickly before.
Also just did a little bit of research, and apparently tiger barbs shouldn't be housed with mollies. Or really anything but their own species. Barbs are known for being quite aggressive. I'd recommend taking the newly bought tiger barbs back tbh. Mollies can be housed with lots more species of fish, just remember that mollies need near perfect water conditions. Be prepared for lots of water changes! I usually do mine twice a week.
Sorry for not being able to find anything on why your barbs may have died so suddenly.