Tank cleaning, what did i do wrong?

A large water change leaves fish showing immediate distress, gasping, lose balance, lie on side, and die. Cause; aged tank slowly consumed carbonate buffer - (low kH) pH slowly drops, toxic ammonia becomes non-toxic (ammonium) and builds up. Then with water change, the buffer is suddenly restored, the ammonium reconverts to its toxic form, ammonia, and the fish die of ammonia poisoning. It's called 'Old Tank Syndrome'... A weekly 20% or more water change with a good gravel vac, and it will not happen again..... Frank

Possible the TDS of the resultant tank water was quite a bit lower than the original tank water which would cause Osmotic Shock. Only way to be sure and avoid this sort of problem is to follow Frank's water change advice or buy a TDS/conductivity meter and use it to ensure that the fish only experience a TDS change of no more than 50 per 1/2 hour. This could take hours and hours depending on the full change.

If you had an ammonia issue, it could have easily been managed using a product like Amquel to ensure the ammonium stayed ammonium even after the pH rose above 6.8.....thats assuming there was any though.....

thanks for letting me know. the angels, clown loach and algae eaters seem to be doing fine..actually for the first time in weeks they seem happy! i will heed the advice given for the future.