I imagine that not being able to get ALL the dirt out of a pool is probably a pretty common problem, so I'm hopeful that those here that have "been there done that" can offer some suggestions.
I have a very small pool which I built for my dog. It's 17' x 8' and only 2 feet deep - approx 2,000 gallons. Can be seen on this thread if you need more information that that.
The pool has a small Hayward sand filter / pump combo unit. 40 some pounds of sand in filter.
I did several searches on this and the most common answer seems to be to backflush while vacuuming to carry the dirt out of the system. I've tried this multiple times and the fine dirt ALWAYS comes back by the next day, so it's not being backflushed out.
I vacuumed (on backflush) about 4 inches of water out of yesterday and the dirt is back on the bottom today like nothing happened. I suppose it doesn't hurt anything, but I sure would like to have it gone!
I'm talking VERY FINE dirt here, like swish around what collects on the bottom and you can't see it anymore. But, it seems to collect together and LOOKS like much bigger dirt when it's settled.
Being that a dog is the main pool user, I do have a sock type extra filter that I use in the skimmer basket to keep the dog hair from getting into the filter. It just doesn't catch the fine dirt. I've researched adding DE to the filter sand, but (from what I've read) it doesn't seem like even that would trap the fine dirt.
I'm to the point where I am considering using a shop vac (remember it's only 2 feet deep, so that's doable) but, I'd probably have to do this quite often and I'm looking for a better solution. The shop vac idea would be a lot of work!
I'd very much like to hear of any other cleaning suggestions to remove this very fine dirt, because I have a LOT of it and can't seem to win this battle.