Greenwater Ph Rising, The PH of my greenwater went from ph7 to ph11(!) in two days. Options

[ed-boy]

I cultivated greenwater outside using a fishless cycle.

I then added 1 small (2cm) fish to a 45L tub with 5% greenwater & 95% fresh de-chlorinated water, however the PH would rise rapidly and become far too alkali: in just 2 days it went from ph7 to ph11! I removed the fish immediately. What am i doing wrong?

Too much sun? (I am based in the UK and had the tub in full sunlight.)
Not a big enough tub?
Has anyone else had this problem?

I am guessing that the variety of algae i cultivated was something like spirulina which actually creates and thrives in an extremely alkali environment. How can i cultivate a more PH neutral strain of algae.

[Ichthius]

All algae consumes carbon dioxide and drives the pH up during the day and crashes the pH at night when they produce carbon dioxide.

Aerate your green water and it will mitigate this to some extent.

Is your tap water soft? It seems to have a greater swing in pH of soft water that is poorly buffered.

The pH swings are very a daily swing and do not seem to harm the fish. What harms them is gas super saturation. You can actually get oxygen bubbles in the fins of the fish similar to the bends but not as bad as the fish can eventually metabolize the oxygen.