I returned home this afternoon after an evening away to find one on my silver tipped tetras dead at the bottom of the tank lodged onto a plastic plant. I firstly thought she'd got stuck but after taking it out and then not thinking too much of it I noticed that I could only count three of the original six and then found two others lodged in places. The tetras were my first purchase about 9 months ago. I have two black mollies, a clown pleco and 5 clown loaches. The mollies were added to the tank 6 weeks ago and they produced fry of which only managed to save two. They are in the tank also but in a net until they are a little bigger to release. I've had an infestation of mini snails which I manually extracted loads during yesterday's tank clean out. I have 4-5 assassin snails added a week ago. Is there a chance I've introduced a disease that has killed off the tetras. They all looked healthy and behaved as normal yesterday. The other fish and remaining tetras all look ok and no sign of white spot.
One of the mollies has a small white spot on her side which I noticed Friday and was keeping an eye on it as it hasn't got bigger, there's no white spots like sugar and no apparent spots near their mouths...but could this be causing a problem? What might it be and the best way to treat.
As usual these things always seem to happen after the pet stores close so I don't have access to medication until Monday evening which may be too late.
Any advise would be great. Ammonia is 0 which was one concern since I did a clean of the filter too but didn't change them or rinse. I haven't done anything different to how I've cleaned the tank before so I'm confused and really hoping I don't wake up tomorrow to find more gone

I've got white spot treatment but what should I use if there is a bacteria or fungus I need to get rid of?
I have a small tank but need to get a heater if I need to quarantine one of the fish.