Don't worry, you can leave sponges running in the tank during water changes. I do it every day. I only rinse them when they start looking dirty, which is about every 3rd water change for me.
To cull a discus means "to throw the fish out into the yard".
You don't need an RO. Use the water you have. Don't be messing around with the Ph either. A novice messing around with water chemistry is asking for trouble.
Don't worry about buying anymore tanks at this time. Learn how to keep Discus alive without killing them first, then you can worry about bigger tanks.
Why buy all these tanks and equipment, only to damage or kill the fish by making mistakes with their care? I suggest you get the tank you have now running successful for 6 months or so with healthy fish living in it before you start complicating things with multiple or bigger tanks. Patience is a good thing to have in this hobby. Take your time and learn how to do things properly. It will save you time, money, and your fish. 
To cull a discus means "to throw the fish out into the yard".
You don't need an RO. Use the water you have. Don't be messing around with the Ph either. A novice messing around with water chemistry is asking for trouble.
Don't worry about buying anymore tanks at this time. Learn how to keep Discus alive without killing them first, then you can worry about bigger tanks.
don't have to yell! I'll get a new test kit.