okay, the advice on this thread seems to be differential by person, and I have to agree with both sides to some extent as too many water changes of too much can have a small impact on the cycle while when doing no water changes when there are is a substantially large amount of build up in toxins it can potentially harm the fish as we are seeing right now with 2 dead fish that are usually very hardy and can withstand a high amount of harmful toxins that some fish may have trouble handling
First we need to find the cause of the problem to prevent it from happening again, some causes could be- the dead fish releasing toxins into the water, under filtering, tap water contains large amounts of ammonia, too many fish, decaying matter, the cycle has been ruined... well the possibilities are endless but the most reasonable ones are most likely what I listed above.
Now we need to keep this problem at bay and take to ammonia down to atleast .25 ppm. and to do this i would add seachem prime, change the water daily and add java moss, and hornwort.