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.