Equipment

There are “ready-to-go” aquarium kits at Petco, PetSmart, and Chewy ($120–$600)-low quality and incomplete 🔥💸

Tank($20-$10,000+):
They say, 'The bigger the tank, the easier it is to maintain.' Larger water volumes absorb environmental shifts slowly, giving you a safety buffer. In small tanks, temperature drops and nitrate spikes can happen quickly.
Petco/PetSmart has 20 gallon long tanks regularly on sale for $40. Better to buy new than risk a used craigslist tank(?)
Waterbox has beautiful well crafted rimless tanks if you want something amazing.
Higher quality glass is more exspensive.
Follow the rimless aquarium's instructions for whether a leveling mat is required.
Think about what you want to stock and get the tank that suits it. If you want a tiny tank- consider shrimp rather than fish.

Tank stand($100-$5000+):
Water is heavy, a 20 gallon tank can be as much as 200-250lbs with water, gravel and hardscape.
Research the weight and get/or build an appropriate stand. Also nice for keeping all the fish stuff in one place.
While considering the stand consider placement- access to water, windows=algae and how busy the space is.

Filter($15-$500+):
It's recommended to get a bigger filter than the gallon recommendation on the box says. If you have 20 gallon tank get a 30/40+ gallon filter. A little extra filtration capacity can provide useful flexibility but also consider if the animals like low or high flow.
Sponge Filter
+ Good for low flow animals
+ Good place for nitrifying bacteria to live
+ Easy, simple, least expensive (might be good to get a battery powered one as a backup incase of emergency)
- Ugly and takes up space in aquarium
- Poor mechanical filtration
Hang on back (HOB)
+ Uses less space inside the tank than sponge
- Needs compatible rim for unit clearance, still ugly
+ Good mechanical filtration
+ Good place for nitrifying bacteria to live
Do not follow the "monthly throw away filter" advised on some HOB devices- ☠️🐟
Instead replace the "throw away filter" with a permanent bio filter sponge and some bio filter ceramic rings.
Never change all your filter media and replace with brand new "clean" ones!☠️🐟
Suggested to occasionally rinse filters in tank water and do not replace until literally falling apart.
Canister filter
+ Provides high flow for animals that enjoy surfing current
+ Holds a lot of bio filter media- sponges, ceramic rings etc.
+ Excellent mechanical filtration
+ Excellent place for nitrifying bacteria to live
- Could leak if hoses are bad (Better to buy new than risk a used craigslist canister?)
- Cleaning means dragging it outside, rinsing trays, every 2-3months(+/-) according to the bioload of the tank
Never change all your filter media and replace with brand new "clean" ones!☠️🐟
Porous rocks (lava rock) act as a "bio filter" giving home to nitrifying bacteria in the nooks and crannies quietly converting ammonia to nitrite and then to nitrate.
Live plants also remove nitrates from the water column. Not all plants are equal- generally floating & terrestrial plants taking CO2 from the air and roots drawing up nitrates, will work faster than fully submerged plants. 

Light($15-$500+):                                                                                                                                                                   Aquarium plants need a color temperature of around a 6500K
Hygger LED lights have sunrise/sunset mode, easy option.
You can get a timer and a cheap 6500K light from hardware store.
Turn off lights at night.🐟💤

Heater($15-$100):
Not all fish need a heater but most tropical fish do need someway to keep the water warm. If you live in tropical zone and keep the room at ideal fish temperature the heater will work less and last longer. Cooler places mean the heater works harder. Good to have a backup heater incase one fails. Also make sure not to cheap out on one that might overheat the fish. Hygger is decent.

Net($10):
Good for catching aggressive or injured fish, and the safest way to introduce new arrivals. After floating the bag to match water temperatures, pour the fish and water through the net over a bucket. This lets you safely transfer the fish into your aquarium without letting dirty store water contaminate your tank.

Syphon($40):
Hygger easy hose to sink attachment.

Aquarium tool kit($15):
Angled long tweezers, scraper and scissors. Good for planting, plant maintenance, algae removal on glass and getting stuff out of the tank without putting your hands in.

1 "Aquarium only” 5 gallon bucket($4):
For fish emergencies, washing substrate, filter sponges, cleaning, whatever.

Power Strip ($7):
For heater, filter, lights etc.

Optional:
Water skimmer($15):
Attached to output of the filter, keeps the surface of the water clean from "biofilm", looks nicer. Many filters have this built in.

CO2 system & plug-in timer($100-$500+):
There are many bundled options. Save money by buying pieces individually. Plants thrive with the addition of C02.- Research!

Additives for plants($12-20+):
A general liquid fertilizer, or specific additive like liquid potassium can make a difference for submerged plants- while fish waste gives them a lot of nutrients some need to be added like iron, potassium etc.

Chiller(if you need one) ($250):
If the aquarium is in a hot climate you may need a chiller- especially if keeping cool water fish. Many fish tolerate some minor temperature fluctuations.

Use this amazon affiliate link to support  AqAdvisor the fish stocking calculator site. I don't have affiliate links but you can help that person.

Setup for 20g Long Aquarium - $654USD
There are “ready-to-go” aquarium kits at Petco, PetSmart, and Chewy ($120–$600)-low quality and incomplete 🔥💸
Equipment: $462
Tank 20g Long($40)
Stand($190)
Filter($50)
Light($60)
Heater($26)
Net($5)
Lid($26)
Aquarium tool kit($15)
Syphon($40)
5 gallon bucket($4)
Powerstrip ($6)

Water Treatment $58
Seachem Prime($18)
API Freshwater Master Test Kit($40)

Decor $134 Substrate, Hardscape, Plants
8lb AquaSoil ($17)
20lb Black sand ($25)
3 Java Fern ($7each)
2 Anubias ($7each)
2 Tall Anubias ($7each)
1 Vallisneria Americana ($8)
Decorative wood ($25)
Decorative rocks ($10)

Use this amazon affiliate link to support  AqAdvisor the fish stocking calculator site. I don't have affiliate links but you can help that person.