Short question - could algae "hide" or grow in retaining wall blocks given how porous they are? I continue to get dead algae on the floor, clean it up, go through the SLAM process, pass the 3 criteria, only to have it very slowly reappear dead on the floor and I am wondering if the retaining wall blocks that I use (edit) to weigh down my ladder(end edit) are its home. I've brushed them, and don't visually see anything. Are they just generally a bad idea?
For the usual suspects, I've brushed the ladder and don't visually see anything there (would I see algae if it were growing there?)
The only light I have is the return eyeball and I cannot remove that without draining down the water level, but this seems like an unlikely source given the constant circulation.
Here is my background for the last few days, in case it helps, I use the recommended test kit:
While my water stays clear and I pass the OCLT and CCs have never shown up on my tests, I continue to see what I believe is dead algae accumulate on the bottom. I vacuum it up, and within several days, it is back. I keep a solar cover on almost all of the time and the cover itself does not appear to accumulate any filth, so there is little evidence pointing to it just being dirt.
I raised to shock level again Wednesday night (17ppm for my CYA of 40), wife tested yesterday around 3pm (20 hours later) and FC was down to 11, so definitely something in there that is consuming it, particularly given that I left the cover on this whole time. I raised back to shock, then tested last night for the OCLT. Tested 17ppm before bed, then 17.5 ppm this morning (assuming a small testing error in there somewhere) so I passed there. I've gone through a similar exercise 2-3 times over the past month.