Aquariums! Inventive way to take your $$?
The answer is, well, yes, of course it is XD But let's look into how it all works.
The best place to start is the beginning. How do you set up your aquarium?
Click "Aquarium" under the "My Gaia" heading. Here you'll meet Barrett, an affable young amateur fish-keeper. He gives you 2 goldfish, 1 arrow fish, an overseer (clam), and a basic aquarium terrain. Barrett has important points to make, so read through his introduction.
The overseer will open once daily (once installed) to give you something. This can range from a 1g item, to a semi-random amount of gold, to fairy wings, so it is in your best interest to check on those fish everyday and to keep them relatively happy.
Fish have limited lifespans, which are longer for happier fish than depressed, neglected ones. There are many ways to make fish happier, some of which I am not 100% sure are more than rumors. They are as follows:
feeding them
playing music they like
having the right lighting
having the right temperature
having people click or "tap" on your tank
keeping the aquarium open and spending time with the fishes (20+ minutes)
Happier fish mean a higher overall tank health level, and that means better items! The correlation is not perfect: fish happiness is apparently important regardless of tank health. Pretty much though, happy and healthy will come together
Tank health begins at around 5%. It slowly rises as you populate the tank with fish and decorations. You can lower it with unhappy fish or an empty tank, although it seems like you have to actively ignore your tank to do so.
Different fish like different things. Yes, it is confusing, and it is difficult to mix and match fish without a lot of premeditation (if you're going to be a stickler about fish health). Basically, there are three things a fish can like/not like: the song, the temperature, and the lighting. If you satisfy all of these requirements, they'll go up 16 happiness points. If you satisfy two, they'll go up 8 points. If you satisfy one, their happiness will stay the same, and if you satisfy none, it'll go down 2.
This is an incredibly in-depth look at fish, including what makes them happy:

You do not have to clean your tank! It only looks dirty when the creepy phonograph is in the process of turning your fish to skeletons (which doesn't damage the fish or the tank in any way!).
You can name your fish and your tank. You can only name fish once, though.
Once fish have been placed in a tank they can't be taken out and their lifespan has begun. Barrett can babysit them for you though, if you'd like. Fish seem to get bigger as they age, so you can tell your younger fish by their size relative to others of their species. The current maximum is 15 fish per tank. You can check your fish stats in the aquarium.
When fish die they leave "drops" which you can open for more free items. These are often commons. Goldfish drop "gold" items and glasses most often. Other fish drop pianos regularly. The following site is pretty exhaustive:

There are two routes I've seen people take when it comes to aquariums. The first is to slowly fill the tank with items garnered from daily chance (various backgrounds, decorations, and fish are easily attainable this way), slowly raising tank health over a long period of time. For those willing to spend a few bucks (or an awful lot of gold, alternatively), there is always Phin Phang, a shop that takes only Gaia cash. Aquarium items have also been available in "bundles" at a cheaper Gaia cash price. This can all be bought with gold in the marketplace if cash is not an option for you. Of course, there is an entirely different category of person, one who just wants an aesthetically pleasing tank full of pretty fish, with cash to spare and no real strategy behind their aquarium.
I personally subscribed to the slow, free accumulation theory until recently, when I sprung for a beginner's aquarium bundle as a Christmas gift to myself redface My overseer went from giving out mostly 100 gold to 1000 gold with the addition of the items (I've yet to add any fish from the bundle, as I am currently only raising cuttlefish, which are daily chance fish). I've also gotten some pretty expensive items (up to around 20k worth commons). And of course the incredibly deflated aquarium-only items. I don't know if it matters, but I stick items in there whenever I get them. My aquarium health is 86.24%, and I keep a ton of cuttlefish in there under natural light and chilly, alternating songs whenever I remember to do so.
I've seen theories that 100% tank health actually decreases the goodies from the overseer. I don't know if there is any truth to it though. 70-90% range seems to be a great spot to be in.
Guides: for your unanswered questions

(good for the basics, lists all the fish and decorations)

Guide to fish happiness! Exactly what it says
Aquarium backgrounds: preview before buying!
How to care for fish in case you needed another good basic guide.
Easy to understand guide to what fish do and don't like. Also tons of info!
So, what are your theories? What has worked for you? Any tips or tricks?
I'm curious if multiple of the same item helps the tank at all.

