Looks like fish are dying

Question
Hi, I have a tropical tank with a small finrot issue.  Today I changed 25% of the water and added the relevant treatment for finrot and water treatment, but since doing the change/treatment alot of the fish are swimming upside down, lying upside down on the bottom, lying against where the water filter goes is and in particular my clown loaches and some larger fish are just lying on the bottom looking as though they are dying (the clown loaches do this regularly I know, but normally move if you go near the tank - on this occasion they look like they are gasping and cannot move).  Also one of the clown loaches appears to be gasping at the top and banging into the tank swimming upside down.  Have you got any ideas what has happened?  I'm desparate as I don't want all my fish to die.

Answer
Hello,
This cannot be related to the finrot as finrot does not do this to fish. Are you sure no other chemicals have entered the tank in the last couple of days? Very sudden deaths or drastic behavioural changes are often caused by dangerous chemicals entering the water.

Because I do not have a detailed description or pictures of the fish, I can only suggest doing daily 30 percent water changes and keeping the gravel very clean. Also, move the filter up towards the top of the tank so it breaks the surface of the water more.

One last thing, Clown Loaches grow very large, so eventually need a tank of at least 240 litres. You will need to accommodate this need for the future.

I hope they survive.
Best wishes,
Kathryn