Dear Simon thank you for your contribution, you are entitled to post what you believe is the best way you see to deal with issues Just as I am, the difference being I understand the science behind swimming pools and the chemistry so no mis information from me.
Now your first point, "the only way to get rid of dead algae"? there is really no other? When people are under pressure to get the pool clear, often with guests arriving they need to have methods which may not be 100% effective but ones which are better than 50% and quick. That is why allowing dead algae to settle and vac to waste is used and used by a lot of people. Sure if you have time then filtering it out is ok providing the filtration is fine enough to catch it so it doesn't just get returned to the pool basin.
"When you have a green pool, put in a bucket of choc" Really, and that isn't mis information? One of the most common reasons for a pool going green is a high level of cyanuric acid stabiliser (CYA) which comes from packaged products like Chlor lent, Chlor multiactions & chlor choc. The only packaged product that doesn't contain CYA is calcium hypochlorite as sold by HTH commonly in France or liquid chlorine (eau de javel) therefore adding a "bucket of choc" (your post didn't describe which type) can make the situation even worse. Choc is a process and NOT a product, choccing a pool is raising the free chlorine level to a point where full oxidisation of everything living in the pool takes place AND holding that level at a sufficiently high level for long enough to ensure all the algae is killed and all disinfection byproducts are oxidised out of the pool. All choc pastilles/granules are the same as a basic galett but smaller so they dissolve quicker, there is no magic, same product but liquid chlorine works even faster as it mixes quicker because no time is wasted dissolving it.
As to slagging off Magiline, I am very grateful to Magiline, Desjoyaux and Waterair, Yes of course they work and providing you don't get into trouble water wise you won't notice the difference. The difference is when there is an issue they take a lot more work to get them back to clean and clear. Take your Magiline system, the brochure would tell your it circulates 50m3 per hour by it's wonderful system. Well it can't, If you look at a swim jet counter current device, these flow 50-60m3 per hour (Badu swim jet) this requires a pump of 3-4 Hp and no filtration in the way. your much smaller pump cannot push that much water in that amount of time due to dynamic headloss and draw it through a filter element, it's impossible due to the laws of physics.
The other points with these systems are that they have a single return nozzle placed very close to the skimmer, so water is returned and doesn't flow up to the far end of the pool and circulate the chlorine to all areas. The water moves around in the skimmer area and is drawn back in leaving the majority of the pool with very little flow. Eventually the water and chlorine will get there but the flow is very poor when compared to a conventional system. This can be tested with some food colouring.
Maintaining a slightly higher chlorine level does help in these pools as it provides more of a buffer against a drop in chlorine in corners etc. Depending on the size and shape of the pool the system can also work marginally better in a curved/hexagonal pool than say the 12m x 5m Desjoyaux I was first introduced to many years ago.
Now I am very grateful to these companies as I said, they help me to earn an income but they also made me question if these things can actually work with such poor filtration (Magilines filtration is better than Desy's) and water flow which again is so poor but they do that at the expense of having larger usually bespoke pumps which cost more to run in electricity are more stressed so likely to fail sooner, what could I do with my knowledge of hydraulics to provide a better flow and far less energy, as the Magiline, Desy and Waterair demonstrate very little flow can still work if all the other parameters are good. That has meant I have developed the most efficient, ultra low cost to run pool flow and filtration setup which only uses 10-15% of the electricity a conventional pool uses saving thousands of euros over a 10y period. More than that the water quality is far better, the chemical usage drops and there is less manual maintenance to do. all thanks to some of the worst designed pools in France.
Back to your sand filter issue Simon, not all sand is made equal, some sand is better than others, I was working on a pool near me a couple of weeks back and the pool is having trouble clearing, I have had a good look and cleaned the sand but the sand in the filter is too course so dirt is just passing through. I should have been back to that by now (sorry Andrew) and replaced the sand with AFM glass. Now I don't know the details of the setup you are struggling with but not all sand filters are created equal either, some have very poorly made laterals which in the case of one pool only had about half of the slots molded cleanly so water flow was too fierce through the remaining ones and channeling of the sand occurred and poor backwash performance meant too much muck was retained in the filter which then meant more chlorine was being consumed, in effect it was a biological filter not a pool filter.
If you post more on the sand filter system giving you trouble maybe we can take a look at it?
In closing, I will just add that we seem to get two algae blooms each year and the next one seems to arrive early August, I have already been notified of two pools going green, so just as a warning to keep your chlorine levels up a bit over the next few weeks.