I agree. A recommended dose excel made my corydoras trilineatus go belly up within minutes of adding it each time. I am very wary now with excel and I never dose the full amount because of that experience, and prefer smaller doses more often.
Often an event that has a negative effect on fish health can take a few weeks to show through organ failure IMO, so if there was an ammonia spike (or whatever) a month ago you might only be seeing effects now. A small ammonia spike (and the following nitrite spike) can be over pretty quickly and may be missed with weekly testing, so its possible that there may not be an active problem now but is the result of something that happened a month ago. However, I would still stick to your current plan of water changes etc. If in doubt, change the water!
I would be changing the water as you plan on doing, definitely cutting back on the excel or changing to a much smaller alternate daily dose rather than on once a week, and using a course of seachem "stability" just to be safe as calculator as mentioned. I use stability every time I start up a tank, even if I am moving a fully cycled filter on to the new tank.