bookfish said:Gravity fed ATO's need nothing more than a sealable reservoir and a feed tube. For small tanks the materials list would be a 2 liter soda bottle, some silicone, and a piece of airline hose. Could cost as much as $7 or $8. For larger tanks a carboy plus vinyl tubing is all you need.
Great advice, Jim. Physics tends to be more reliable than aqualifters, powerheads, and ATOs too!
One note... don't oversize the reservoir. It is easy to make the mistake of putting in a bigger reservoir so that you can do less maintenance. But do so and an accident will cause too big of a swing. I recommend you only make it big enough for 3-7 days of topoff.