In theory, sand could wear out. Constant wear could cause the grains to get too small. Probably only take about 10K years or so.
But then they would just wind up in the pool when they washed out.
What happens is the sand bed "channelizes". Actual term. It gets sticky and clumps together. The water then cuts channels through it and so you lose the filtering.
It can be cleaned, but its usually cheaper and easier to replace it.
Its usually caused by various oils from sunscreens and just general skin secretions.
I have helped change a few sand beds over the years. I only remember one that we figured really needed it. Darn stuff came out in clumps and did not even want to wash away.
Personally I have always use a DE filter, because I like my pool water cleaner than sand can get it.
Putting a small amount of DE in a sand filter can work, but its tricky and can clog the sand.
Baitfish2 talked me into helping him change his sand twice in less than a month. First time, he said he was sure it was channelized, so I went up and helped him dump a large amount of what looked like perfect sand.
About 2 weeks later, he called and said we put in the wrong kind of sand. The bags said it was for pool filters I thought, but OK.
We dumped another tank full of perfect looking sand and replaced it with identical sand.
Two weeks after that, he called and said he was going to get more sand. I asked why. He said his pool was still so cloudy he could not see the bottom.
I said "floc your pool". He misunderstood, and we had a pungent conversation.
Finally I got him slowed down and told him to go to the pool supply store and buy some flocculant.
What happened was, he had an algae problem. Killed the algae, but it broke down into particles too small for the sand to filter, making the water cloudy.
A flocculant is a chemical that binds to the particles and causes them to clump together into larger particles. Then they sink to the bottom and you can vacuum them up with the pool vacuum.
A week later and his pool was clear.
As for the tank, I have a couple of ideas, but they all are either too much trouble or too expensive. I'd either glue it and don't worry about it for the next few years, or buy a new one.
If you have new sand, it should be good for several years unless you have lots of sunscreen or oily swimmers. So, no need to open it. Just make sure everything inside is good to go and glue that sucker up.
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