Filling speaker stands: sand, lead, steel, rice. Full, half, etc?

As long as there is no wave action, you should be fine. One could or could have drilled a bigger hole.

As a metallurgical engineer who has spent much of the past 40 years looking at corroded metal,  yes, wet sand will cause steel to rust.  (I assume the stand/tubes are steel.)  ideally, you want to dry the sand out. Practically, this can’t happen unless you remove the sand. Theoretically, you could bake the sand filled tubes or put them in a lab vacuum, but neither of these is realistic.  Partly because the moisture would have to escape through the holes- and with small holes and slots of sand that is many inches away from the holes it would take along time for the entire sand to dry out.

fortunately, indoors under controlled environment- the rust process will be very slow, taking years to produce a light rust skin on the inside, and probably decades to affect the structure/thickness of the tubes,  and the sound would only be affected if there was serious thinning of the walls.  If the inside of the tubes was painted (which is doubtful unless they dip paint the tubes rather than spray them), rust will be much slower, so slow you can ignore it.  Adding a rust preventative (wd40) to the sand could help, but only if you could shake it up enough to coat the entire inside, which seems impossible, so not worth the effort.
So, assuming the sand was “damp” and not dripping wet, you should not have a problem.   If it were me, I would just not worry.  Perhaps start off your listening with Neil Young’s Rust Never Sleeps.

@meiatflask thank you very much for the response. I decided to just leave it alone and yes, the sand was damp... I dried them out but only for an hour under the sun, filled the tubes with the half wet half dried sand.. I shaked the stands this morning and surely i could hear the sound of sand moving inside. Probably wouldn't worry too much about it now