Can somebody help me design a cost effective resivor? Here is my general design:

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here
I dont think as of now, that this deisng will work, but hopefully you will get what im trying to do. Im trying to avoid a big resivor, while being able to add water without having to open a closed system. Or do you guys suggest an actual resivor? I saw a picture of this one guys system, it kinda looked like a water treatment plant (big resivor). I am open to any suggestions, even some good resivor suggestions. I have a sepp case for my pump and resivor, so space isnt really a factor.