The use of backwashing

[Doggie]

My 25000 L inground salt water pool is near overflow and i need to bleed it to reduce the level. Obviously I can either backwash or waste. My questions are :

1. Which to do ? also can you backwash too much and damage my glass filtering material ?.. I was told about once a month, as I have minimal rubbish entering the pool.

2. Should I vacuum first ?

3. If required, would you shock the pool or add more salt.

Thanks

Doggie

[waterbear]

If your combined chlorine is .5 ppm or less (ideally 0 ppm) and you are maintaining the aporopriate free chlorine level based on your CYA (stabilizer) level then you do not need to shock. A properly maintained salt pool usually does not need shocking since it is constantly being shocked within the cell 

Only add salt if your salt is too low. Keep your salt in the manufacturer's recommended range. Both too low salt and too high salt can shorten cell life or damage the salt cell 

I ghave never heard of too frequent backwashing damaging the filter bed, whether sand or glass. Backwash as often as your pool needs it whish is usually weekly to monthly. If the pool is kept covered and used less than once every two weeks then monthly should suffice. If it's used daily then weekly.  If it's not covered I personally would not go more than every two weeks. However, this also depends on whether you filter is oversized, sized to pool capacity, or undersized (which does happen).

If the water is too high I would vacuum to waste,backwash if needed, add more water if you remove too much, check salt and add if needed, let run 24 to 48 hours and then rebalanced water as needed.