Personally I wouldn't go back to tap water but I would at phosphate to your water. You were definitely depleting your water of PO4 with the changes you made which gave the dinos the niche to bloom...not crazy at all. Tap water will contain a lot of other things that you won't want to be adding to your tank. You can get bottled PO4 for a reasonable cost .I know this is going to sound crazy. When I first started my reef I was using tap water. I never had a Dino problem I had hair algae. So I switched to ro water and running gfo and my hair algae disappeared. Fast forward 8 months I stated to have I now know is Dino's. Been dealing with it ever since and can't have much clean up crew either.
I'm setting up a refugium and going back to tap. We'll see how it goes.
Yeah I hear ya. I always try to confirm with a third test kit just in case. But I usually keep acros that are VERY sensitive to alkalinity changes. It may not hurt for you to raise both PO4 and NO3 to see if that will help...PO4 = .06ppm NO3= 0ppm and I discovered the alk. issue because I bought a new Hanna alk. tester 1 month ago. I had a bad kit previous to this one.