Great resource here helping each other - have often read it but only posting now.
I have a major problem I'd be most grateful for any help with. This pool was one my late father built (structural engineer), so it has extra special meaning to me. Its a 9,200 gal / 35,000 L, SWG, concrete/plaster (marblesheen is the term here in Australia), sand filter.
From last year/season and continuing now, I've had an endless problem with what looks like black algae (but its not limited to just small spots, and keeps reappearing in these patches that easily brush away but reappear within 1-2 days and get darker). I've been to my local pool shop and spent hundreds of dollars on their 'recommendations' - everything from copper based algaecide, multiple phosphate starvers, pool clarifier, tried bombing it with liquid chlorine to a high level (15+) also tried cal hypo bombing, kept pH lower around 7.0 to help fight the algae also. The copper algaecide helped only SLIGHTLY, but still, almost every day or two I'd be going and brushing the algae away but it just continued to reappear. Even with the chlorine very high, it still reappears and seems resistant to it. During last season I've tested the water weekly and everything has been within proper levels (CYA about 60, chlorine always at least 6-8, alkalinity always around 100 +/- 5-10.). Currently there is no copper algaecide left in the pool just for reference, CYA is about 40. Pics are taken all recently (last few weeks).
A few observations I've noticed:
- the level of calcium (CH) in the pool has repeatedly been 350+ in every test. I know concrete/marblesheen pools need higher calcium so haven't worried about it. The SWG often gets calcium deposits on the plates (less when I turned it slightly down over winter). In just a few spots in the pool, there is like calcium buildup (a slightly mound) and sometimes the black algae/patches will appear near these. Probably only 2-3 of these lumps though.
- there is two small spots in the deepend that look like actual black spot algae. I've bought triclor pucks and will brush them using a wire brush while leaving a puck on each spot for 30mins-1 hour this weekend. Nowhere else are there actual tight small black spots.
- on the outer edge of the pool, the black algae patches quite often appear in areas where the pool surface looks rougher. However, also is areas perfectly smooth it also appears sometimes darker (see pics).
- the shallow end pool steps have a metal handrail, and one of the bolts is slightly rusting (small patch of rust around it thats underwater). I mention this in case it has some relevance.
Please see the pics (ignore the few leaves near the pressure relief below to help identify it. I'd be most grateful for any help or guidance here - if anyone can work out what it is, its you guys vs the pool shop, clearly they have not been able to solve the problem at all. Thanks so much, Andrew.