I use this for my 5gallon (18 litres). It does a great job, my only suggestion is to buy a better filter media for it, the white balls it comes with are essentially just ceramic, not doing very much for the tank.
https://www.amazon.com/dp/B07SFCRGG6/ref=syn_sd_onsite_mobileweb_0?ie=UTF8&psc=1&pd_rd_plhdr=t
A light and sponge filter would be nice!
To setup a sponge filter you need an air pump: https://smile.amazon.com/Tetra-Whisper-Easy-Aquariums-Non-UL/dp/B0009YJ4N6/ref=sr_1_6?crid=1AR81YYWFJ7TQ&keywords=aquarium+air+pump&qid=1579189396&sprefix=aquarium+air+%2Caps%2C182&sr=8-6
There are some really cheap sponge filter options, and they work just fine, but I like how this one has 2 sponges and 2 ceramic ball media areas, I just clean 1 of the 4 "zones" each week when changing water (rinse the sponge or ceramic balls in old aquarium water): https://smile.amazon.com/Aquaneat-Sponge-Aquarium-Ceramic-Airline/dp/B07SFCRGG6/ref=sr_1_15?keywords=sponge+filter&qid=1579189516&sr=8-15
Nice little light with day and night settings: https://smile.amazon.com/ECtENX-Aquarium-Light-Lighting-Color/dp/B07FNDPG4L/ref=sr_1_6?keywords=beta%2Blight&qid=1579189718&sr=8-6&th=1
Edit: Also, betas like the water about 78 deg F, so you might want a heater: https://smile.amazon.com/Aqueon-100106251-Preset-Heater-50W/dp/B00Q78MMT6/ref=sr_1_3?keywords=5gal+heater&qid=1579189843&sr=8-3
If you dont have well water please use water conditioner
API Stress Coat Water Conditioner
I put in stress coat or stress coat + zyme after a water change or when a fish loses color or looks like it has an ouchy
[API master Water Test Kit] (https://www.amazon.com/dp/B000255NCI/ref=cm_sw_r_cp_apan_glt_fabc_2KGRHRGN4H778VSS338H)
The master test kit is great to see of the nitrate or ammonia is too high
Great for looking at all the chemical levels and properties of your aquarium water
Will let you know if your tank is to warm or to hot
For tropical fish like plecos
I use this when I bring new fish home or when doing a water change to make sure the tap water or Reverse osmosis water is not to hot or to cold
Laifoo 25ft Aquarium Vacuum Gravel Cleaner Fish Tank Cleaner Siphon Water Changer
If you dont have a water Siphon you can use a garden hose or PVC flex pipe from lowesnirnget a water changes from Petco or petsmart you will need a water dechlorinator if its town water if it's well water you should be ok
Great for water changes and easier on your back and floors
5ML pipettes
I love using these when I use stress coat or other liquid medicine or for plant fertilizer
You should have aquarium only gear that you never use soap or bleach on if you need to clean something only use water and distilled white vinegar
Aquarium Co-Op Measuring Glass
Measure glass for fish meds, liquid ferts, or aquarium salt
Beneficial bacteria
Aqueon PURE Live Beneficial Bacteria and Enzymes
Live beneficial bacteria is usually placed in your filter chamber under the pad so it can grow and Culture on the cotton or sponge filter ( beneficial bacteria is microscopic so if you can see growth something is wrong)
Beneficial bacteria for new filters or new filter cartridges
Fritz Zyme 7 Live Bacteria (Freshwater)
Bottled beneficial bacteria
slow release starter colony beneficial bacteria
Plants help remove and reduce nitrates if you got frogbite, Salvinia cucullata (Asian Watermoss) , duckweed if you dont mind removing some very week.
Plant holder like lucky bamboo or pathos plants
Poth-O-Carry™ Plus: Pothos Holder with Open Face Technology
You should have a secondary bio filter and never clean the hang on back filter the same week as the secondary bio filter should be a sponge filter or a a ziss moving bed filter
Sponge filter
AQUANEAT Double Bio Sponge Filter, Quiet Aquarium Filter with Ceramic Media Balls
These are both air driven so you will need
[air pump]
Air stone
Aquarium Air Stone, Ultra Silent Deep Dissolving Oxygen Diffuser