This is the part you want: John Guest 1/4” Angle Stop Valve (ASVPP1LFD) It's around $15-20
The John Guest 3/8 in. Comp x 1/4 in. O.D. Push-to-Connect Angle Stop Adapter Valve is manufactured from white polypropylene and equipped with EPDM O-rings utilizing John Guest's push-fit technology to
www.homedepot.com
Turn off the water, flush a couple times to drain the tank, then disconnect the line from the tank to the supply. Be ready to cath some drips. Slip the new valve in place, retighten, and voila! Not too tight. All these connections are compression fittings so snug but not TIGHT. A bit of teflon tape helps too.
The valve lets you turn on and off the system as you need without interrupting the toilets workings. I'm not sure which ROsystem you have but it should be similar. I fill the jugs in the tub and use buckets to catch the runoff and irrigate the outside gardens with it. If you have't got a place to dump the water it can just go down the drain. Unfortunately the inexpensive systems produce a lot of waste but it is still magnitudes cheaper than bottled. And it's there when you need or want it! It can also be used in your ultrasonic humidifier if you have one.
Here is how mine is plumbed in and I suspect you could do something simlilar. This is in our guest bathroom and I run the lines into the tub.
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Slip the line behind the toilet and onto my RO system mounted to a cheap board. It needed to be removeable in case of guests and this works pretty well.
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Ok hope that helps.
PS - I would highly suggest taking that line off the toilet tank and bringing it to the store so you can match up the fitting sizes. I suspect it's 3/8" but if it's an older place.... well... Good luck!