What fish could I add, if any? And filter help

[Apple]

I am new to this so I would appreciate any advice on anything.
I have a 65l tank with 2 platy, both female and 6 gold tetra, they are happy together. I was wondering if I could add any more fish/creatures to the tank or is it quite full for 65l? If I can, is a betta a bad idea? I know they’re not very good community fish but my other fish are quite peaceful and I wondered if it would work. However one of my female platy does have a large top fin, I’m not sure if that would make a betta more aggressive. If a betta is not a good idea are there fish/creatures that would suit my tank? (I’m not set on a betta)
My tank has:
-sand with gravel underneath
-some decorations
-live plants (thinking of adding more)
-heater, water is at 24-25°C usually
-two filters, as i recently bought a new one because I’m getting rid of the older one, the older one is called interpet and it’s an internal cartridge filter, and the new one is called Aqua flow 200 or something like that. The reason I bought a new filter was because the old one slowed down a bit and i was told this new one was better. Inside the new one however, there is just a large black sponge whereas the other had stuff like a green algae sponge and a white sponge-thing with activated carbon inside, and these stone like things with holes through, like beads, I think its called biomedia. (the black sponge could however contains these maybe?)
Im not a filter expert lol
Anyway thanks for reading

 

[WRWAquarium]

Personally I wouldn't add any more fish to the tank. 65l isn't that much water really. I wouldnt advise a Betta, may work may not but its a risk.

Quite importantly make sure you keep that sponge from the old filter and put it in the new one. That is worth it's weight in gold with its bacteria keeping the water healthy for the fish. Never clean it with chlorinated water just tank water. The flow reduces the more detritus blocks the sponge and it needs squeezeing out in old tank water.

Myself I would run both filters. I would also do away with carbon and algae pads and replace with more sponge and ceramic biomedia. Ultimately you want as much surface area for good bacteria, carbon isn't that useful and the algae pads probably useless.

 

[Apple]

Personally I wouldn't add any more fish to the tank. 65l isn't that much water really. I wouldnt advise a Betta, may work may not but its a risk.

Quite importantly make sure you keep that sponge from the old filter and put it in the new one. That is worth it's weight in gold with its bacteria keeping the water healthy for the fish. Never clean it with chlorinated water just tank water. The flow reduces the more detritus blocks the sponge and it needs squeezeing out in old tank water.

Myself I would run both filters. I would also do away with carbon and algae pads and replace with more sponge and ceramic biomedia. Ultimately you want as much surface area for good bacteria, carbon isn't that useful and the algae pads probably useless.

Thanks,
so just sponge is good enough to keep the water clean? Does the bacteria in the filter clean it?