H20 changes vs. SA - which is better?

I refill my pond with nutrient-filled tap water, which I'm sure adds to my
SA problem. Eventually I may install a rainwater collector, but until then,
what do you think: do the benefits of water changes outdo the harm they may
do to my SA population?

I had been doing 5-10% water changes weekly, but I wonder if I reduced this,
would overall pond health not suffer but my SA go down? My pond is full of
plants, floater, etc. all doing well, add K, magnesium (low in my area) and
BZT weekly. Healthy, non-fed fish, happy frogs, healthy plants, but healthy
SA, too. Hmm, opinions?