HI there. I am in same boat<or Tank> if you will>. I have so far found you need a Electronic float switch from hardware store or ebay, I have saw for around 3 bucks. then you also need a small pump with in the specified head distance, again 3-4 bucks on ebay . Next a tub or holding tank<s>of sorts . I have 2 , one for RO
water to fill to 22L from my RO
unit, and then a 3 dollar pump is hooked to a float switch set in the second bucket <Same Size, Walmart, both for 15 bucks>. set at 22L, then another 3 dollar pump heading to my Main Tank attached to yet another float switch and that float set to the lvl I want to keep tank at. Then you need an overflow , DIY on u tube with PVC
easy peasy for just one tank. I took this pic off another site I had read. HE said it is ok to use his knowledge. Then just time the flow from tank with ball valve so your water leaving the tank is same as what is coming in. VERY IMPORTANT. Put a quarter inch drill hole about another quarter inch below desired water lvl in case of Power failure even if this design works with out having to re-siphon I do not trust it . Of course if just a auto top off I would use the smallest PVC
Piping, or instead of 2 intake like the design I would just use one. The only issue is The salt mix in second bucket before it goes to tank. I was thinking I will see if I could use a old Auto Fish feeder and somehow rig it to dump the salt in daily but 22L is around 3- 4 cups . I am stuck there so far hehe. Research a bit on you tube there is alot of Designs and FLow rates etc. I found this the easiest and simplest. Just the salt adding who or what will do it to keep salt content in tank. Otherwise just straight RO
water is going in which salt lvl will drop in a few days. Hope that starts you on your way .