15k Gallons InGround plaster pool. FNS 48 Filter.

Two part problem here.

Replaced filter grids about 2 yrs ago. Pool used to run every night from 8pm to 5am. Then at some point it was running for 2-3 hours and flow would trickle to a stop. I have cleaned the grids checked everything is snug but this problem continues. I recently cleaned and installed a new filter O-ring. Ir ran all night and seemed to really improve, not as good as it was 10 year ago but better than about 2 years ago. Only noticed a slight 2..3 point rise in pressure. Then I decided to vacuum and it a quarter ways done it lost suction pressure. Backwashing helped it only slightly, not enough to vacuum. Now running it the same problem is back, of flow back starting low and coming to a stop within an hour in fact. Also noticed when pump starts running it throws some air bubbles back to the pool for a minute or two then it clears. Am at the end of my wits with this issue and need suggestions.

Secondly I really need to vaccum it so decided to acquire a pump and setup an external pool vacuuming pump while I try to figure out the issue above.

I got a Century 230/115v regular pool pump  I would like to use as an external pool vacumming pump. I am looking for help on the correct way to safely wire the pump. 

1-It's rated 115/230v. I think 115v is convenient since I can just use a regular outlet, correct?
2-That 115v does not reduce the pump's performance compared to 230v, correct?
3-If 110v is aok what gauge wire/extension cord is acceptable? Same question if it has to be the 230v mode.
4-I would like to position an on/off switch. What amp switch is acceptable or will a regular household wall switch be ok?
5-Please instruct/draw etc the correct way to connect wiring.