Green algae on glass after water change

[Blenny]

I don't get it. The day after my water change the glass gets covered with tiny green hair algae.

Been doing water changes with tap water for the last year without any issues. Since I managed to lower my nitrates considerably (15-20 ppm down from 40) in the last month I figured this was now the time to switch to RO/DI after witnessing this last week.

But same effect this week with RO/DI.

RO/DI water measured 0 nitrate and phosphate.

My tap water also btw.

Salt is Instant Ocean.

Anybody have a clue what is going on?

[Blenny]

I have been thinking maybe iron is limiting my algae growth.

I stumbled on a old thread reading on algae scrubber and a very experienced reefer suggested that.

So when I do a water change I might replenish my iron.

I will have to get an iron test kit to confirm that out.