This will be difficult to decide, I don't want to have to worry about leaks like the first person who replied said, but I also don't want to worry about burning out the motor if it runs dry. I also don't want to have to refill it with a hose more than a couple times per month in case I forget to turn it off before leaving town for a couple of weeks. Now I'm a bit more inclined to go with the autofill and take my chances that it may leak someday. The water flow rate should be too low to have any concern for it flooding the patio since the faucet feeding the line should only need to be opened a crack to provide enough water to cope with evaporation. Maybe there is a way to set the base so that it will be obvious if it develops a leak.

Sure, it's done all the time. You'll need a reservoir to hold water, a pump in the reservoir, some sort of feature for the water to cascade over with the appropriate plumbing, and support for the feature and camoflage to hide the reservoir. Look up "bubbler" on the web and you should find some examples.

Joel, epoxy is inert and more stable timewise than concrete or even cross linked polyethelene. Keep in mind that you want to use a Marine grade epoxy like West Systems. If you do not want to use epoxy, if you clean and prep the tank really well, wash it down with muratic acid, you can then coat the tank with a bonding agent, and trowel on a parge coat of acrylic and fiber reinforced topping cement. Mix it up thick and stiff. You need to trowel it, not pour it. To get a smooth surface, keep trowelling and polishing the coat until it is rock hard. That 120 gallons that Canadians use is just suburban use, and does not include farm irrigation water. The trick is to use the 80 or 120 gallon figure as an estimate to determine cistern size. Four homes sharing a 600 gallon tank, even though it is refilled during the day, is small if there are more than two people in each home. You may want to consider irrigating at night, or adopting less consumptive irrigation methods like drip.

In my neck of the woods the guniters will clobber you on the price to shoot a monolithic concrete shell. They have a minimum and this shell would come in at 2500 just for their service alone. I'd build this platform with two sided forms laying the bottom of the inside form of the wall 6" above the dirt. Have the excavator dig a footer beam all the way around the perimeter 6 to 12" deeper and place some footer beams across the short side on your 6,12,18,24. Place the outside form first, tie steel on a tight schedule, 4each #4 boxed beam and walls/floor #3 8"oc, then form the inside wall. Call a concrete company and order 3500psi redimix with the smallest aggregate they offer, I'd ask if they have 3/8" sifted. I'd guess at about 6 to 7 yards of redimix concrete, my cost would be about 500 to 600 for the crete. I'd hate to guess the subcontractor costs in your area and would suggest taking quotes from an excavator/formsetter, rebar contractor, plumber, electrician, and a brick/tile mason. One pair of VGB compliant main drains and a surface skimmer are a must, the bubblers can act as the sole return but I'd also suggest a wall return to aim flow at the skimmer. "A" profile floor returns will do so so for keeping the floor semi clean. You could also use an old style 5 port water valve from Polaris in conjunction with a Leaftrapper, it'll act much better as a debris catcher better than the skimmer. You'll need to utilize spa lamps for the reduced lumens due to your small size, large lamps will fit in 12" of water but offer way to much light. For a hands on contractor, this type of job has the potential to hit a home run, but farming it out to all the different subcontractors will get expensive due to the minimum charges from the subs. I hope this helps. See ya, Kelly