The timer you may be most tempted to buy is possibly a digital timer because you're probably like me and thinking "how high tech would that be?!". Actually this would probably be the worst option to buy for your aquarium setup. If your power ever goes out though (like at my house it happens about once a week) then you may find that a digital timer will reset itself completely when power is lost or when the unit is unplugged. Ultimately, I find a digital timer to become the most annoying, difficult, and least desirable for use on aquariums... unless of course your digital timer happens to come with a battery backup inside of it (is there such a thing?).
Now you've probably came across some aquarium timers that are available at your local pet store. Often times these can be overpriced timers and generally come on a big power strip... although you more than likely don't want all your aquarium setup connected to some timer strips unless they have outlets that stay on continuously (which is typically what you would want to run your filter off of). These range from around $15 to $44 at PETCO, but you could opt to buy regular non-digital timers for less at Walmart or even your local drugstore. Also make sure if it has a power strip (or you use a power strip for that matter) that it has surge protection so it doesn't ruin your equipment that's plugged into the unit, and I personally recommend having one that you can change the settings for the various plug outlets on the grid because you may not want your lights and air diffuser, etc all working at the same intervals. Also, you may find these do not have enough outlets on the power strip for your setup/s and you'd end up having to buy yet another unit or have another basic power strip on hand.
The cheaper house timers which are non-digital timers I find to be the best option. If your power cuts off for a short time it doesn't mess up your timer settings too drastically and it can be easily fixed with a simple twist and re-alignment of the timer. This timer device may only give you 1 outlet but, if you buy a duo/trio/etc plug attachment to connect more than 1 plug into the timer or even attach it to a power strip then you'll add to your outlet issue. There are also a few different non-digital timers, I personally prefer one that lets you adjust it by 15 or 30 minute intervals on the device... where as some may only come with one set of on/off clips for the entire day (which is like the timer below on the right side of the image).
|My current setup I am working on.|