People get real !
The practicality and versatility of an Arduino build is what attracts most people to this kind of projects.
How many fish or corals of more than $20 you bought and they died ?
The biggest investment is time ! To build one of these controllers you need less than $100., but ... more than two or three weeks of work if you know what you are doing. If not, go buy a commercial one. To build this one you need hardware & software knowledge as well and regardless how much I wouldn't like to say it, it is not a DIY project for everyone.
So, if you are to invest the time do it the way you take advantage of the full potential of such kind of project. You will alway need more digital pins, you will always think about more complex scenarios and the more you do it the easier this hobby will be.
I built one of the first controllers (reefprojects.com) a year or so ago and it has been working flawlessly since. I have expanded two weeks ago using a Mega, 16 controlled outlets, pH, ORP, temp, two float switches, etc. and I could not be more satisfied. It was more than $100 though, but way less than a commercial version.
Always remember that what satisfies you today will not tomorrow (Kano model).
hope it helps,
Marian
PS. No intention to argue with anyone, just a point of view.