Just want to mention that it isn't advisable to run a waterfall on a timer. Of course, you might just be joking. But if you weren't, there are a few reasons why many people suggest against it. For one, if you have a filter at the top of the fall, or anywhere n that line for that matter, the off on cycle can cause debris in the filter to become unsetteled and flow back into the pond. Not always, but sometimes. Also, the benificial bacteria that live along the path of the water fall will die if they dry out. These bacteria are critical in the conversion of toxic ammonia and nitrite into much less toxic nitrate. Another reason is that the pump and waterfall are circulating the water and adding oxygen to it. You want to keep the water well circulated and highly aerated for the fish health, and because the bacteria I mentioned earlier function best at high levels of dissolved oxygen. Sure, turning the pump on for half an hour and off for half an hour or something like that won't be catastrophic, but I still suggest against it unless you have a smaller pump moving the water. The thing I would really suggest against is running it all day and turning it off at night. Plants create oxygen during the day but the use it up at night. Combine that and no water circulation and you could be looking at low oxygen levels , which can kill fish. I'm not saying one night of the pump off will cause them to die, I'm just saying fish can die from low levels of dissolved oxygen.
