Well I know that those water clearer things don't usually work. It's easier to address the problem than to solve one side effect of it.
You don't have to replace your filter cartridges but you should swish them out in already siphoned off tank water when you do your water changes if you're sure that it's fully cycled.
I'd definitely suggest getting a test kit of any kind when you can. Petco and Petsmart will test your water for free if you bring in a sample.
Could you list how many fish you have and the species please to make it easier. Just a list is fine ^^
When were the fish added and in what order? If the betta was first and the others somewhat recently added it's probably territorial aggression. When adding new fish into a betta's tank you need to completely rearrange the decor which makes it seem like a whole new tank to them.
Your tank has a good amount of decor but I would add some very tall silk plants to break the line of sight a bit better. The more hiding places and just filled the tank seems the better usually.
This is how my 10g betta tank is set up. It's him and a mystery snail. They get along fine but I tried to crowd my tank so there was plenty of hiding spaces. Bettas can sometimes be stressed out by open spaces and this varries fish to fish. Some do well in large tanks and others need 5g or less to feel secure. Each betta is different so it's really hard to say what's best without knowing your fish personally. Some do well in community tanks and others don't.
This tank has a goldfish and a pleco in it. I provided lots of hiding for the timid pleco and lots of silk plants to hide in for him.