A friend has a fiberglass pool. It's seven years old, and he's never changed the water, not even a little. And since he uses stabilized cholorine, there's no telling what the stabilizer has built up to. He lost about six weeks of swimming time last year trying to get the algae under control.
But he has been warned never to drain the pool more than a foot or so because of the water table level - it will pop up out of the ground just like a boat, which would be a disaster.
I think it should be relatively easy to change out essentially all the old water with new, and to do so without dropping the water level in the pool at all, and without wasting any water. All you need is a membrane that separates the old water from the new.
So you go to Home Depot and buy a roll of 0.35 mil plastic sheeting (12'x400') for $24, and some shipping tape, and you tape together what looks like it might be a liner for the pool, or at least that big - doesn't have to be exact, just not too small in any dimension. Then you turn off the pump, anchor the edges of this liner around the edge of the pool deck with a few bricks, and pile all the excess out onto the pool surface. You want to do this on a calm day.
You set up maybe four or five garden hoses as siphons that suck water out from under the liner, and start them draining the pool. You put maybe two garden hoses as supply hoses on top of the liner, and over time adjust one spiggot so the inflow rate is about the same as the outflow, so the water level stays constant.
Over a day or two, the liner will slowly expand as the new water comes in and the old water goes out, and will fill out to conform more or less to the bottom and sides of the pool. It doesn't have to have any strength at all - all the forces on it will be applied very very slowly and gently, and it will never have to actually "hold" water. It's just a diaphanous membrane that always stays at the boundary between the new water and the old.
When it's done, turn everything off, gently remove the sheeting liner, turn on the pump, superchlorinate, and you're done.
If you want to get fancy, you can run one of the fill hoses through an auto-fill gizmo. Then you don't even have to check on the fill rate - the water level will be automatically maintained.
I think this will work. Is there a problem with it, or something I haven't considered? Whadayathink?
I would appreciate any comments.