I was thinking that there might have been a overload in the filter from the inital set up which would require the filter to be cleaned more often and never getting the chance to build up good bacteria (re: cleaning twice weekly. Washing away muck and bacteria) and the ammonia could have been slowley building up till it became a problem.
I might be wrong but when I have set up new tanks and add fish too quickly the filters become dirty and clogged but if I add the same amount (or more) to the same tank over a longer period time I never have a problem. We did have a problem when first set up the filters needing to be cleaned weekly but now we have three times the fish in there and it is not needing to be cleaned as often