Young Betta Isn't Eating

My sister has a young female betta. She has not been eating much, and I think her growth is getting stunted. Also, she is losing some of her color. I haven't tested the water yet because the last testing strip I had was a dud, but we are going to get the water tested at a pet store soon.
The tank is a 20 gallon with a floating algae boom. Some Acu-Clear has been put in the tank, but I read some of the reviews on Amazon and although it has killed some fish, it doesn't seem to harm bettas. When it does kill fish, it happens a lot faster. This has probably been going on for over a month, ever since the betta was adopted.
The aquarium has been getting weekly water changes. There were no 75% water changes due to the issue, though.
So far, we have tried changing her food. Her old food seemed like it was high quality, with the first few ingredients being bugs, but we are trying some pellets that have garlic in them.
If we really have to move her, we have an empty 10 gallon which was used for rodents, a 55 gallon which has 4 pearl gouramis, a female betta, 6 silvertip tetras, a bristlenose pleco, 2 dwarf crayfish, a bunch of bladder snails, and 4 assassin snails. The other 2 aquariums that are set up are definitely too small for more fish and also have female bettas.
Thank you for reading!

Any chance of a picture and short 60 second video of the fish?
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What symptoms does the fish have?

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If you have a green water problem, it's caused by single celled algae growing from excess light and nutrients. The easiest way to deal with green water is to reduce the light and nutrients, or add lots of live plants to use the light and nutrients.

I couldn’t figure out how to attach a video, but I got a picture. In the video I took, she just stayed in the same position as the picture for 18 seconds and then swam only enough to change her position. Thank you for helping! Also, her only symptoms are clamped fins, a lack of appetite, color loss, and lethargy.

The fish looks awful.

Stop adding chemicals.

Wipe the inside of the glass down with a clean fish sponge.

Do a 75% water change and gravel clean the substrate every day for at least 1 (preferably 2) weeks. The water changes and gravel cleaning will reduce the number of disease organisms and chemicals in the water and provide a cleaner environment for the fish to recover in.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.

Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use them. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens.

Increase surface turbulence/ aeration when using salt or medications because they reduce the dissolved oxygen in the water. The extra aeration helps the fish recover.

If there's no improvement after a couple of water changes, add some salt (1 heaped tablespoon per 20 litres of water). Keep salt in there for 2 weeks. But try water changes first.

Colin_T said:

The fish looks awful.

Stop adding chemicals.

Wipe the inside of the glass down with a clean fish sponge.

Do a 75% water change and gravel clean the substrate every day for at least 1 (preferably 2) weeks. The water changes and gravel cleaning will reduce the number of disease organisms and chemicals in the water and provide a cleaner environment for the fish to recover in.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.

Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use them. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens.

Increase surface turbulence/ aeration when using salt or medications because they reduce the dissolved oxygen in the water. The extra aeration helps the fish recover.

If there's no improvement after a couple of water changes, add some salt (1 heaped tablespoon per 20 litres of water). Keep salt in there for 2 weeks. But try water changes first.

Click to expand...

Thank you again for helping! I got the water tested and the person at the store said there was some nitrite, but it wasn't urgent. He also asked if there was a heater. There is, but I decided to check it because I remembered it being too high before. It was completely off. I thought it was broken at first. The water dropped down to 70 degrees F. Should we still do the water change, or is it probably just the heater?

You should do a 75% water change and gravel clean the substrate any day there is an ammonia or nitrite reading above 0ppm, or a nitrate reading above 20ppm.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.

Colin_T said:

You should do a 75% water change and gravel clean the substrate any day there is an ammonia or nitrite reading above 0ppm, or a nitrate reading above 20ppm.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.

Click to expand...

Okay. I'll make sure to do that today. I really appreciate your help! My sister and I were pretty worried about her fish. I already have some aquarium salt from when another betta was sick if we have to use it.