The sponge or media pad in the filter could have carbon in. If it does, the carbon will remove the fertiliser from the water. This will happen until the carbon is full at which point it will stop removing anything from the water. As a general rule you don't need carbon in the filter. Carbon is a small black granulated substance.
The sponge or media pad could also have zeolite in and this will remove ammonia until it is full. If there is zeolite in the filter it will stop the beneficial filter bacteria developing and you could have ammonia and nitrite problems.
Zeolite is a small white granule.
You need to find out what is in the filter pads. If there's zeolite or carbon in them, add a sponge from a different brand of filter and leave it there for a month before removing the carbon/ zeolite pad/s. You can buy sponges for different brands of filters. get one that is the same size or slightly bigger than the filter pad and add it to the filter. You can use a pair of scissors to cut the sponge down if it's too big. Sponges don't need replacing unless they start to fall apart (they last for years) and won't have any carbon or zeolite in.
-------------------
Fix the timer on the light so it's on for the same time every day and doesn't have a short day on Sunday.
I assume the filter is run continuously (24/7)?
If the filter is run continuously it is fine to turn the air pump off at night.
-------------------
I would remove the 3 spikey plastic plants so they don't damage the fish. It shouldn't be an issue due to the Betta being short finned and the plants being really close to the bottom, but if the fish gets caught on the points it could damage the fins.
-------------------
There appears to be new growth on the Anacharis. The rest of the cuttings are dying or dead and should be removes if they are soft and mushy. Anacharis does not like sudden changes in temperature or light and if it was grown outdoors in a pond (that's where they are usually grown) and then put in an aquarium with colder or warmer water and less light, they usually die. Any plants that survive the shock will normally be fine and do well once they have recovered.
I would remove the string holding the cuttings together and let them separate so they have better water movement around them and to reduce the chance of infection spreading from the damaged tissue to the healthy cutting. If they end up touching each other after that it's not as much of an issue compared to being closely tied together.
-------------------
Due to the number of live plants in the tank (two) and the fact one is low light and slow growing, adding a brighter light to the tank probably won't make much difference. I would look into the plants you buy and try some easy to grow plants like Ambulia, Hygrophila polysperma, narrow Vallis and water sprite (Ceratopteris thalictroides). The water sprite floats and makes a great habitat for Bettas who can rest on the branches just beneath the surface. The other plants get grown in the substrate. If you get too much water sprite you can plant that in the substrate too.