billewing,
Very possibly your problems are a direct result of fertilizing...explosive algae and weed growth following fertilization is something I have experienced and in hindsight should have expected, in spite of expert recommendations to add fertilizer.
Removing the fertilizer, by flushing out the pond, would seem to be a way to get things back to "normal".
One of the things I've learned about ponds is that things that work for some folks may not necesarily work for other folks in different situations. The pond maxim that says you should fertilize to increase fish populations may apply in some situations, but the side effects can be devasting. You may have experienced one of them.
