Help with pH and TA please

[Nick]

I have a 12,000 gallon pool; the water supply is hard and our problem is usually limescale buildup. However, from last autumn the pool water pH inexplicably dropped.

pH used to be high, but it is (I think) so low now that some testing kits don't even register a colour. Also, I never knew to test for TA; I have just started and it appears to be very low (around 40)

The water supply is still alkaline

Is there any possible explanation for this drop in pH? The only change in pool conditions is that there are a lot of dead insects in the pool every morning (there never used to be)

I have started to add Sodium Bicarb to bring up the TA.... is there anything else I should do?

Many thanks for any help you can give

[Buggsw]

Using a lot of pucks? Have any water features running all the time, ie negative edge pool, fountain, waterfall, etc?

[Nick]

The only pucks in use are cloro only, and we only use them when the chlorine drops really low. We haven't got any water features.
I have now added about 5 lbs of sodium bicarb, and the pH indicators are heading in the right direction (back up to 7.0), but TA is still low

Also, presumably because of getting a low pH, we now have black algae on the walls - not much of it, but enough to be visible. Our place is holiday accommodation in Andalucia, Spain, so you can imagine we want to keep everything as clean as we can for our guests

If I get TA and pH back in order, and scrub the walls, will that be enough?

[Buggsw]

Black Algae is difficult to treat and usually comes back unless you acid wash. Could it possibly just be another stain?

Brushing will help open up the protective layer that is found on the black algae.

If it was on the bottom, you could put a chlorine only puck right on top of the spot for a few hours, but the side walls, that's a little hard, to do.

You could try brushing and using liquid chlorine to schock at a very high level for 3 or 4 days. You would need to keep it up to 25 or 35 ppm during that time and run your pump 24/7.