Weak new mollies

[myguppypuppies]

So I recently got a group of mollies, 7 of them from a breeder. When I got home I acclimatized them for 30min and released them into the tank. They all swam to the bottom and grouped together behind a large ornament. I figured they might be shy or adjusting so I wasn't too worried, my tetras had hidden for a day or two too. However, now its been just over a week and they're still acting weird! And its not just one or two, its the whole group. One of them died right away too!
I've talked to the previous owner and he suggested a 30% water change which I had already done just in case. I'm still waiting for him to tell me his tank's temperature in case the mollies are used to a different temp.

Water parameters.
ammonia - 0.0
nitrate and nitrite - unfortunately, don't have this tester
PH - 6.7
temp' - 26 C or 79 F
Hardness - I'm not sure. I just use tap water + conditioner and the tap water is supposed to be 11.6 grains per gallon

2. A full description of the fishes symptoms:
- All 6 of them hover slightly above the bottom or even rest on the gravel, staying upright but unmoving. They only act normal at feeding time or first thing in the morning they seem fine and active.
- One or two of them has a clamped fin
- At least they are still eating
- No white spots, fungi, or anything else as far as I can tell
-One of them dances weirdly in front of my tiger barbs, but its just the one so I don't know if its related to whatever is affecting them.

water changes
- every 2 weeks, 20-30%

chemicals and treatments:
Small amounts of aquarium salt as a tonic, added during water change

Tank mates
2x tiger barbs, 4x ember tetras, 1x small pictus catfish (5") and 1x pleco (8")

Tank size. 20 gallons

 

[FJB]

I don't know the reason for the un-wellness or the death.
Although domestic (aquarium bred) mollies are very adaptable, mollies in general like higher ph and alkalinity. Some of them live in near full seawater at least seasonally. The low pH in you tank (6.7) may be an issue. Unfortunately, the tetras (although also adaptable), like the opposite: softer, lower pH than the mollies. So although not impossible, not the ideal community.
What is he water from the tap, after settling down for a while. If it is below 7, you may want to add some coral, shells, or other carbonate materials in your tank. And you are correct that getting information on the water characteristics the mollies came from would be valuable.

For the future, it is not a good idea to introduce new fish directly into your tank. It is best to set up a quarantine container to receive new fish, and to hold them for a period of time (I do at least 4 weeks, usually longer), but people do a range of durations.

Hope this helps. Good luck!

 

[FJB]

Also, I just realized it is a 20gal.
Too many fish for that tank, IMHO.
Just the pleco and the pictus cat are by themselves, too much for the tank.