I have the Krystal Clear C1500 filter which comes standard with this 18' x 48" pool. It pumps 1500 gallons/hrs and the pool holds around 6000 gallons, I believe. If your water is blowing out of the filter and into the pool, it has to also be sucking. I will say that even with my big filter, you can barely feel the water with your bare hands sucking through. Let me know if you have additional questions I can help with!
Hi! Yes, your ground should be level. The spot I put mine is SLIGHTLY sloped, which makes one side of the pool an inch or two deeper than the other side. If it's more lopsided than that, the pool side will fall down and a bunch of water will spill out, especially when it rains hard.
