Looks like fish are dying, please help

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
Hey Lesley,

Sounds to me like your suffering a high ammount of CO2. This is typical with fizzy tabs, and poor water circulation. If you used the jungle medication tabs (which i prefer) you could have spiked the CO2 level. A few small live plants like java ferns, bamboo, or a large potted plant or two will help drastically remove nirtite and nitrate, as well as change CO2 into O2. The added plants provide hiding places for your fish as well, and will relieve stress.

Check the water quality as well, you could have amazingly high ammonia levels, that are poisoning your fish. If so, Amquel and NovAqua are the best water conditioners around. NovAqua is lightly medicated to prevent bad bacteria growth, and AmQuel removes ammonia and bad nitrite/nitrates!

One of those two is most likely the case, if your water tests out okay, and you already have live plants, please ask a followup and we can consider things in further detail.