new fish continue to die

Question
I have a eclipse 12 gal tank.  It was doing well and then suddenly all the fish starting dying.  I clean everything added new fish and they died within two days.  I cleaned the tank again, left it for a month and added 4 guppies.  Withing two days they died.  I have tested the water, it is okay perhaps a bit soft.  The temp is at 78.  I am feeding tetra fish flakes.  I have bettas in different 3 gal tanks and use all the same supples for water changes and they are fine.  Even the snails I have in the larger tank died.  It is possible that the decorations may be toxic?

Answer
Hello Linda,

How are you today? I think that you need to set up your eco system I have seen this problem before, now this can vary a lot because you could have a disease in the tank. The first thing I would do is turn the temperature up to about 80. Then I would go purchase a water supplement that adds good bacteria to the water that is need because without it fish well die within days. After that I would clean the tank out extremely good to try and eliminate disease being a problem. From there you should just have to fill the tank with fresh balanced water and add the supplements and add the fish

Thanks for your time,
Michael M.