0. Research
Before buying an aquarium, you must do research.
If you just bought your fish and aquarium on the same day in the last week, return the fish because fish-in cycling is challenging and not recommended for beginners.
First, you should ask yourself if you are truly ready for the responsibility of owning an fish and an aquarium.
Maintaining one can take up time and money.
A good beginner aquarium is about twenty gallons.
Larger aquariums are easier to maintain than smaller ones because their environment is more stable.
Make sure that you have room for your tank.
Water weighs in at ~8.34lbs per gallon.
That doesn't include decorations, equipment, and substrate.
Take a trip to your LFS (local fish store) and find some of your favorite fish.
Then, go home and research them, finding which ones you would like to own.
Research, research, research, I can't hammer this in enough.
Most aquarists, including myself, believe that "the number one cause of premature fish death is lack of research".
Twenty gallons is ideal for the beginner because it can host quite a few different fish.
However, the tank should be setup for the fish, not the other way around.
Some fish, such as goldfish may need 25-30 gallons or larger.
Make sure to understock your tanks.
A good tool for checking how stocked your tank is is aqadvisor.com.
For beginners, I would suggest targeting a 70% stocking level with their tool.
A couple items that you will need are clean towels, a new oral medicine syringe, and new plastic bins.
Get these ahead of time from other sources as they have very expensive versions at the pet store and you will be needing them.
If you don't want the clutter of many bins, the Seachem HydroTote is a nice alternative.
You will still need a few for collecting dirty water and for new fish.
1. First shopping trip to the pet store
After thoroughly researching your fish, it's time to go back to the pet store.
Start by choosing an aquarium.
I personally prefer acrylic to glass.
The bundle kits are nice, but you will usually need to upgrade the filter and/or lighting.
Then, buy substrate, decorations, and plants.
Some fish, such as loaches prefer sand, while others, such as goldfish need larger river rocks because they can choke on gravel.
Decorations are pretty much up to you, but make sure your fish have plenty of hiding spots.
When choosing plants, be aware that they have their own special needs.
I would start with a beginner plant such as elodea or java moss.
Both of these plants should do alright without CO2, fertilizers, or strong lighting.
If you did not buy a starter kit, you can get one of the cheaper lights.
Many commonly sold fish need heaters.
If your tank comes with a heater, great.
If not, and your fish requires tropical temperatures, then the Eheim Jager heater is good choice.
Next, you will need to acquire a filter.
If your tank comes with a filter, buy another filter rated for your tank and use both.
If not, buy filter(s) with a total rating of double your tank volume.
For example, if you have a 50 gallon tank, you could buy a 100 gallon filter, two 50 gallon filters, or any other combination that is around 100 gallon rated, or larger.
If general, sponge filters are best for very small tanks, or certain breeding setups, HOB (hang on back) filters are great for aquariums between 5 and 60 gallons, and canister filters are best for aquariums in the 40-500 gallon range.
In addition to heaters and filters, certain, more advanced fish, also require chillers, UV sterilizers, and powerheads.
If you are lucky enough to find yourself with a very large aquarium (over 500 gallons), a DIY filter is probably your best bet.
Most filters come with media.
You can keep the sponges and other biological media.
However, ditch the carbon, and other chemical media in favor of more sponges.
Activated carbon works, but is mainly just a money-making gimmick.
You can keep it for a quarantine tank discussed under 'fish' or emergencies.
The only chemical media that is actually worth it is poly-filter.
You can buy a box of cheap activated carbon for QT, but it is not needed in your main tank.
In fact, it even can cause complications with the nitrogen cycle, leaving your aquarium hooked on carbon.
Finally, there are accessories.
You most definitely need a dechlorinator and food.
For a dechlorinator, almost all aquarists use Seachem Prime, as it is cost effective and also removes ammonia, heavy metals, and other toxins.
Dechlorinators make tap water safe for fish.
For food, pellet and wafer types are the best.
Live and frozen foods can care parasites.
Live foods are also considered cruel by many.
The common flake food is not ideal because it can expand inside fish, causing bloat.
My favorite pellets are Hikari Micro Pellets, and Aqueon Pellets.
As for wafers, I use TetraVeggie Algae Wafers Extreme.
These are 100% veggie and algae based.
For fish that require more animal matter, different wafers are also available.
You will also be needing a fine-meshed net, at least twice as long as your longest fish.
In addition, I would recommend a thermometer, filter cap for HOB filters, and an aerator.
You should also get a siphon or gravel vacuum.
Don't get the type that connects to your faucet, as you pump chlorinated water into your fish tank.
The last accessory is water testing.
There are generally two different types, strip and liquid.
Liquid is more expensive at first, but lasts so much longer and is much more accurate.
Now that you have all of your supplies, you are ready to go set up the aquarium.
2. Tank Setup & Cycling
Find a place to setup the tank.
Keep it away from windows that are in direct sunlight to avoid algae problems.
Fill the tank with your substrate.
You should have about 1-2 pounds per gallon.
Then, place your decor where you want it.
Fill up the tank with tap water and dechlorinate it.
Wait about 5-10 minutes and add plants if you have them.
After that, start all of your equipment.
Add a source of ammonia to your tank and you have started the nitrogen cycle.
Monitor your parameters and keep adding ammonia until you have nitrate.
Never let ammonia exceed 10ppm.
During the weeks in which your tank cycles, you can research your fish more and print out some maintenance logs.
In addition, test your tap water to make sure that everything is in check.
Depending on your fish, your pH should probably be between 6.5 and 8.5.
If it is outside that range, don't panic, most fish from stores (not wild caught) can adapt to pH's, ranging from 6.0 to 9.0
If your pH is still out of the range, let it sit out for 24 hours to let the gasses dissipate and test it again.
The most dangerous thing to fish is pH swings, not improper pH.
Never apply pH changing chemicals directly to aquarium water.
However, you can use driftwood, crushed coral, and rocks to change it.
If you feel the need to change your pH, apply the pH buffer directly to tapwater, not to the tank.
3. Fish
After you have nitrate readings, wait until ammonia and nitrite are zero, or perform consecutive water changes until they are.
Now it's time to get your fish.
Prepare a clean plastic bin by lining it with towels.
You will place the fish bag in here.
Go to your pet store and buy a few fish.
Don't get a ton, so as not to overwhelm the beneficial bacteria in your tank.
Make sure that they look nice and healthy.
Don't impulse buy.
If you see some new cool looking fish, you can go back and research them later.
Also buy some airline tubing to acclimate them.
If you want, you could by an acclimation kit such as this one instead.
Bring home your fish and acclimate them to your aquarium.
To acclimate them, place them in a clean bin.
Cut the bag and gently pour out the fish.
Place one end of the airline tubing in the aquarium and start a flow by sucking on the end.
Tie the other end to slow water to a couple drips per second.
Keep adding tank water to the fish bin until is is double the original volume.
Then, take out 75% of the water from the bin and fill until double again.
Take a paper cup and scoop the fish into the aquarium.
These are better than nets because they don't scrape the fish.
You will only use the net for taking fish out of the aquarium.
Make sure to test your water daily for a few weeks.
If there are any signs of ammonia or nitrite or if nitrate is over 30ppm, do a 50% water change.
If ammonia or nitrite is still above 0.25ppm or nitrate is over 30ppm, then keep repeating the 50% water change.
To change water, siphon it out into your bucket or bins.
Then dechlorinate new water and add it to the aquarium.
Ater a few weeks, if your ammonia and nitrite are zero, and your nitrates are rising, you can go get some more fish.
Place these fish in a large bin or spare aquarium (Quarantine tank)for at least 1-2 weeks and look for any signs of disease.
You can add some activated carbon to help keep the water clean.
Perform a 50% water change every 1-2 days, but don't share equipment with the main tank.
Some common ones are white spots, blackened or bloody fins, and external worms.
If diseases show, treat them accordingly.
After at least 1-2 weeks disease free, add these fish to your aquarium in the methods described above.
Repeat until you have completed stocking your aquarium.
4. Maintenance
Maintenance consists of filter cleaning, water changes, and feeding.
Every three months, or if filter flow is impeded, take out half of your filter media and gently rinse it in tank water out of the aquarium.
This will remove some debris, and hopefully keep most of the beneficial bacteria.
Water changes are pretty simple and have already been mentioned under the 'fish' section.
As for feeding, feed once a day and start small.
After while, you can feed a bit more, until you get a feel for how much to feed.
Many people state a few minutes, but this has a few flaws.
For me, pellets take 30 seconds, and wafers take 2-6 hours.
Pellets, flakes, and sticks should be less than 2 minutes and wafers less than 12 hours.
You can feed once or twice a day.
If you feed more than once, cut portions in half.