PLEASE HELP & EXPLAIN!!???

Did you remember to dechlorinate at the water change?
Is the tank cycling or mature?
Any chemicals near the tank? (Decorating, cleaning etc)

 

Welcome to TFF.

We will need more data in order to help. Naughts mentioned some of this, but can you also give us the test result numbers [we cannot assume what you mean by perfect, we may see them differently]. Ammonia, nitrite, nitrate. Also GH and pH. What substances were added to the water...if conditioner, which one, etc. Is the temperature normal, and what is it?

 

I’m sorry to hear of your difficulties.

I agree with Byron. We need more information here to help. Your parameters could be a contribution to your problems.

Best of luck.

 

Do a 75% water change and gravel clean the substrate immediately.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.

It's unlikely the water change from 3 days ago did this. If there is chlorine/ chloramine in the new water it affects the fish within minutes of being added to the tank. Fish that have been exposed to chlorine will go to the surface and gasp. They can die a few minutes later or survive for a few days. But they will have lots of trouble breathing.

What did you do to the tank last night?
Do you have any creams, anti-bacterial soap, oil, grease, paint or anything else on your hands when you feed the fish?
Do you use an air freshener, perfume, deodorant, smoke or do anything in the room that could create fumes?
Do you have buckets that are used purely for the fish or do you use any bucket in the house?