Reddit mentions of SLSON Aquarium Sponge Filter Corner Filter Biochemical Cotton Fine Filtering for Fish Tank up to 21 Gallons

Good! They'll definitely need filters though, and that may be part of the problem. As for what kind to get, I'd recommend looking into sponge filters.
They're super easy to clean, cheap, and very reliable. They also hold a lot of good beneficial bacteria for your tank, which is great for your fishs health.

All you need for one is the sponge filter itself, some airline tubing, and an air pump. The sponge filter can usually be bought at local fish stores, or online, and the rest can be bought even at bug retailers like walmart or meijer. I'll link some amazon listings for visual reference, and find the best price I can too.

https://www.amazon.com/SLSON-Aquarium-Biochemical-Filtering-Gallons/dp/B07PF7T1QZ/ref=mp_s_a_1_25?keywords=sponge+filter&qid=1572619104&sprefix=sponge+&sr=8-25

https://www.amazon.com/Penn-Plax-Aquariums-Flexible-Standard/dp/B0002563MW/ref=mp_s_a_1_26?keywords=air+pump+aquarium&qid=1572619157&sprefix=air+pump&sr=8-26

https://www.amazon.com/Tetra-Whisper-Easy-Aquariums-Non-UL/dp/B0009YJ4N6/ref=mp_s_a_1_4?keywords=air+pump+aquarium&qid=1572619198&sprefix=air+pump&sr=8-4

To clean them, which you'll only need to do like maybe once or twice every few months, just fill up a clean bucket with your tank water while doing a water change, put sponge filter in the bucket, squeeze it out, put back in tank, done. No need to buy more cartridges either.

As for decor, I don't know what you've got, but if its artificial, plastic plants and hard decor with sharp points can also tear and rip your bettas fins. If you do artificial, I'd recommend soft silk or silicone plants. If I'm being honest though, I always recommend live plants over fake, but I know that's not much of a possibility for everyone.

If you do want to try live plants, some good beginner ones are anubias nana, anubias frazeri, and java fern. All of these plants do not get buried, and if they do, they will rot. They can be tied to surfaces, or can have a small rock tied to the bottom to make them sink, but they will root themselves.

Java moss is great for bettas, they'll love to sleep in it. Water lettuce is a really pretty floating plant too.

Lastly, I'd HIGHLY recommend doing some research on the nitrogen cycle. Since you're setting up brand new tanks and moving the bettas once they're set up, you'll be doing a fish in cycle. This process is crucial for fish keeping, and good knowledge to keep your animals happy, healthy, and safe. I'll try to give a quick run down for it though.

The nitrogen cycle is the process where your tanks build up beneficial bacteria that break down waste from your fish and fishs food into ammonia, nitrites, and nitrates. The bacteria are needed to maintain a healthy tank and grow on surfaces such as gravel, decor, plants, heater, filter, tank glass, but they are not in the water itself. And a filter is necessary to keep beneficial bacteria because it circulates water, provides aeration, and has a ton of surface area for them to grow in.

Ammonia and nitrites are harmful and deadly to your fish. Nitrates are less deadly, and are what you're aiming to get to in the cycling process, but you still don't want a bug buildup of them either. The absolute most important thing for cycling your new betta tanks for the next few weeks is to be on top of your water change game to keep your bettas safe.

For the first 2 weeks, you will want to do 40-50% water changes every other day. After that, for another 2 weeks, water changes every 3 days at 40-50%. The next 2 weeks, water changes every 3-4 days at 30-40%. After the initial month and two weeks of water changes, you can move onto water changes once or twice a week at 30-40%. It sounds like a lot, but it will keep your fish safe from ammonia, nitrite, and nitrate spikes during the cycling process.

Some good things to keep on hand to help during cycling is some Seachem Prime, or Seachem Neutral Regulator. These are all in one water conditioners that remove chlorine, chloramine, heavy metals, AND detoxifies ammonia and nitrites for 48 hours (not a replacement for water changes though. Those are still very much necessary)

https://www.amazon.com/Safe-250-g-8-8-oz/dp/B0002A5WOC/ref=mp_s_a_1_6?keywords=seachem+prime&qid=1572620732&sprefix=seachem+&sr=8-6

https://www.amazon.com/Seachem-116030601-Neutral-Regulator-250gram/dp/B000255P9E/ref=mp_s_a_1_1?keywords=seachem+neutral+regulator&qid=1572620778&sprefix=seachem+neu&sr=8-1#immersive-view_1572620800767

And a great way to kick start your bacteria bloom and speed up cycling is with some live nitrifying bacteria. I use topfin, and a big bottle is about $10. But api is great too.

https://www.amazon.com/API-Nitrifying-Aquarium-Maintains-Biological/dp/B006YG12F6/ref=mp_s_a_1_2?keywords=top+fin+live+bacteria&qid=1572620840&sr=8-2

And, if you need more information on the cycling process, feel free to reach out, or look up "nitrogen cycle" in google, or check out a very helpful video by KGTropicals called "everything you need to know about the nitrogen cycle."

https://youtu.be/es7spyo5RJ4

Apologies for long post, but I hope this helps you and your little dudes! ☺️

He really needs a filter, you can probably find a tiny sponge filter which all it requires to run is an air pump (which you have) and it can still make bubbles for him! Bettas like jungles, I suggested live plants because they help with the water quality. You need to fish-in cycle the tank, the airstone is okay but you really need a filter to circulate the water through. When the tank is cycled the filter will have enough beneficial bacteria to convert ammonia created by fish into nitrite (also toxic) then into a safer chemical nitrate which is less toxic in larger quantities but we want to keep it below 20.

Heaters:

Tetra HT Submersible Aquarium Heater With Electronic Thermostat https://www.amazon.com/dp/B000OQO69Q/ref=cm_sw_r_cp_api_i_-FRNDbBWFSS7A


Marina Submersible Heater for Aquarium, Mini, 25-watt https://www.amazon.com/dp/B00AFELT92/ref=cm_sw_r_cp_api_i_gJRNDb8MRN195


Hydor 25W Submersible Glass Aquarium Heater - Original Theo https://www.amazon.com/dp/B0006JLPG8/ref=cm_sw_r_cp_api_i_NKRNDbH01Y034

Maybe this filter would fit? It shows dimensions but idk what your tank is like - it would go in the corner -

SLSON Aquarium Sponge Filter Corner Filter Biochemical Cotton Fine Filtering for Fish Tank up to 21 Gallons https://www.amazon.com/dp/B07PF7T1QZ/ref=cm_sw_r_cp_api_i_BMRNDbDEQF4GE

This is also super important-

API Freshwater Master Test Kit 800-Test Freshwater Aquarium Water master Test Kit https://www.amazon.com/dp/B000255NCI/ref=cm_sw_r_cp_api_i_iNRNDbAPV8KBQ

Heres a link to prime:


Seachem Prime Fresh and Saltwater Conditioner - Chemical Remover and Detoxifier 100 ml https://www.amazon.com/dp/B000255PFI/ref=cm_sw_r_cp_api_i_HNRNDbDRHJ1G1