The tank is 10 gallon. The temp is 70 degrees. I’m not sure of the numbers, the guy just said they were right where they needed to be. The old fish died from me putting too much algae fix in the tank when I was having a bloom. I acclimated the fish by setting the bag it was in in the water for an hour to get it used to the temp in my tank.
It sounds like the main issue is that there was something that got into the water and poisoned the fish. It wasn't removed when you changed water after your fish died or if you didn't change water, it was not consumed by your bacteria bed and is probably still present. What exactly that was /is would take very expensive testing by a real lab so the end result for now would be to strip the tank down, rinse everything off and reset it up.
Things to keep in mind:
*While 70 degrees is okay for goldfish, 70 degrees is too cold for fish like the Ember Tetras. They should be in water from 75- 82 degrees. (But 70 would not have killed them outright, it would make them sick first.)
*As Aiken mentioned, a 10 gallon tank is too small for any goldfish specie long term.
* Acclimating fish: you only need to float the bag for 10-15 minutes for the water temps to equalize. You can read the rest of the process here:
https://www.aquariumcoop.com/blogs/...event this from happening,about 30–60 minutes.)
( There a a few ways of acclimating fish but each way is not the best for every situation so if you do any research, look at more than one way. )
*Aerosol sprays should never be used anywhere near your aquarium. When using an air pump, if you can smell what is being sprayed, so can your air pump. That's how even a tank with a lid does not solve the spray problem. You also want to be careful with floor cleaners or wiping the tank down with any cleaners other than water. The product "Windex with ammonia D" has killed a lot of fish tanks over the years due to the ammonia getting sucked into the water via an air pump.
This is also why kitchen fish tanks are not very successful due to oils or fumes emanating from the frying pan. If you can smell it, so can your air pump. ( I repeated this because it is that important.
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Hope this helps.