Debi,
If your water is good (after trying 3 sources I would think one or more shoud be), changing 25-30% should not be a problem. It should be reducing the nutrient level because what you siphon out should have more than what you put back in. Is there any chance that the blooms are occurring at the three week intervals and not because you changed water? You could change your interval for water changes. Perhaps try two smaller changes a week apart, then two or four weeks apart. See if the blooms occur with the smaller closely spaced changes and if they continue to match the water changes or stick to the three week interval.
I am going through a diatom stage now with my new DSB. I have not been changing much water. In the morning they don't seem too bad, but by night time, it is pretty ugly. I believe (and hope) it is a phase that will pass. I will change some more water, but not anything major. I think I will add some Caulerpa to try to use up nutrients.
What causes your blooms to end? Do you siphon it out, or do they just go away? Do you have any measurable nitrates? Mine are still at zero, but the diatoms grow anyway. How many times has this occurred? Two, three, ten? I wish I had better suggestions for you. It may be that for a couple months the tank will be prone to diatom growth as the DSB matures. How is your pod population? Mine was great until I added a little Sailfin Tang. He ate almost everything on the surface of the sand and rocks in just six days. He also ripped out numerous Yellow Polyps. He is no longer in my tank.
I wish you the best of luck with it.
