I had a oil drain plug on a riding mower that was very frozen. I tried wd 40 oil but later switched to an oil such as Nano penetrating lubrication to get better penetration with the lubricate. I also used a 12 to 18 inch long steel pipe that I put over the box and open wrench to provide additional leverage. It was hard to move but with both the penetrating oil and pipe leverage I was able to get the plug loosened.

Yep that's it ...A pair of large channel locks will do it if you can get to it. turn right to tighten and left to loosen...the plug I gave a link to will replace it,,, Rusty

Remove the hydrostatic. If you have a high water table this is the time to find out how high before your pool pops out of the ground. Keep pumping the water out if it keeps filling. Before plaster, if the main drain is not staying dry, install a standpipe in the hydrostatic hole. When ready to fill, remove the standpipe and install a NEW hydrostatic valve. If the main drain stays dry after removing the valve, you could replace the valve with a plug but I am always very leery of doing this. Usually if the last guy bought a hydrostatic valve he had a reason.

I've got the same filter and when I was going through pool school the PB rep told me to remove that plug to drain the tank before removing the filters and I was like "You've got to be kidding me!! How the heck can I even get a wrench on that thing, and then get it back on without cross-threading the plastic threads on it??" Then I pointed out to him that there is a hose bibb installed on the plumbing that is well below the tank seam, and why can't I just open that bibb up to drain the water down low enough to remove the canister top and pull the filters? He looked at it thoughtfully and said that indeed he couldn't think of why that wouldn't work. And that's exactly what I've been doing, LOL! It works great. Hopefully you have a bibb like that on your setup as well, if you do then give that a try and leave the bottom plug alone. Don't forget to open the valve on top of the filter canister to let air in as it's draining down.