There are a surprising amount of things that can be automated for a complex aquarium. With open-source development boards being very cheap and eaesy to use, there have been many projects surfacing to tackle some of this automation.
In the realm of aquariums there is a massive amount of variation not only in the contents of the aquarium, but in the complexity of maintenance. Reef aquariums, or salt water aquariums containing inhabitants that would normally thrive in a reef environment, have long been a symbol of complexity and beauty in home aquariums.
Another recent area of enthusiasm are "planted aquariums," visually appealing freshwater aquariums that harnesses strict lighting, nutrient, and carbon dioxide to create works of art. If you've got a little time to kill, you should really take a peek around the AGA aquascaping contest website to see how fantastically amazing some of these aquariums can look. If reef tanks are more your style, you can find plenty to gawk at.
"Destiny" by Sim Klan Hong -- First-place winner of the 320Litre and larger devision of the AGA aquascaping contest 2013.
To keep either of these kinds of aquariums, you have to control a list of things. Your lighting needs to be timed and even mimicking natural light cycles can be beneficial for cultivating some more difficult plants. One common practice to promote explosive plant growth is to introduce higher levels of C02 to the water -- this too can be done in cycles to optimize the effects -- as well as water temperature and flow. Mimicing temperature drops or rainfall can have positive effects on some fish. Feeding and additional nutrients can also be automated.
With all of these variables for automation, you can imagine that there are already existing automation systems available. Unfortunately these systems are quite expensive, and often closed-source. With the popularity of open-source development platforms like the Arduino, Raspberry Pi, and BeagleBone, people have been making all kinds of control systems. These are typically more tine intensive, but ultimately cheaper and more expandable.
I've collected a few projects for you to look at. These are mostly labelled and described as "reef" controllers, but would work equally as well for a planted freshwater aquarium. Click on through to the next page to get some inspiration or possibly see how to build your own.