You could also try to bring that phosphate down by a combination of fast
growing or, ideally, floating plants and less frequent water change
since it seems phosphate comes from your water supply.

If you live in a more rural area, or a small city, you could have your
rainwater tested to see if it is fit. It has some risks, but I've heard
of various degrees of success using rain water, but DON'T use it if you
live near a moderately or highly populated city (too much contamination
from industries and transport)