So excited! A friend asked my advice about starting up an aquarium for his kid. Here goes. (Note that these are my opinions on a freshwater aquarium, based on my experience. Others may have different ideas, suggestions. Feel free to comment/elaborate.)

Hardware:

• Bigger tanks are more forgiving and stable than smaller tanks, probably don’t go smaller than 20 gallons, unless you’re just going to get a betta (I love bettas! If you get a betta, don’t go smaller than 5 gallons no matter what any fish store person tries to sell you.) A glass tank will hold up better over time than an acrylic tank. Avoid weird shapes and tanks with narrow openings. No matter how cool it looks, if you can’t easily reach every area inside the tank it will be a bear to clean.

• Filters. I’ve had three and by far the easiest to use and maintain is AquaClear’s hang on back power filter. It’s a great little work horse. If you really get into this hobby, then I’m a big fan of the Eheim canister filters.
• Lights: Depends. If you don’t want live plants in your tank, you don’t need a lot of light and several tanks come with a hood that contains a light. If you become obsessed with plants, then lighting becomes important. Homework will be required. There will be math. And you will spend money. Here’s a good primer.
• Plants: It’s possible to get some realistic looking fake plants. For live plants, java ferns work even in low light and you don’t even have to plant them. Just tie them to a rock and they’re good. I had fun at one point adding a floating plant, water lettuce.
• Livestock: This is the  hard part. Everything above is influenced by what kind of fish you want to keep. Goldfish are easy, hearty, dirty fish. Angel fish are gorgeous but aggressive. Bettas are slow moving, beautiful and entertaining, but hard to keep with other fish. Figure out what you like and then build the system that supports them.
• Substrate: It could be gravel, it could be sand, it could be soil. Depends on the livestock and the flora. Please don’t choose some horrible fake crap or I will have to reassess our friendship.
• Placement: In figuring out where to put the tank in your house, here are a couple of things you should consider. To keep a healthy tank, you need to do a 10-20 percent water change every so often (I do it once a week, but you could do it twice a month I think with no problem). You can buy a hose with a faucet fitting that will allow you to both suck water from the tank and refill it, or you can do the water change with a bucket. that along with cleaning the tank will mean that there will be dripping and spills. So placing the tank where you can do easy water changes and where you don’t mind spills (I put down a big towel  when I’m working on my tank) is important.
• Warning: It’s a big glass box of water and lots could go wrong.
• Keep it simple: If you have no idea whether you will like having an aquarium and keeping fish, I would suggest getting a five-gallon tank and a betta. They are easy. It’s not a huge investment and you can figure out whether it will be interesting for your kid or not. You can often find a used tank at St. Vinnies. to help keep the price down.