scales sticking out

-My betta is in a 18 litre aquarium. (4.75 gallons) with a filter, a heater, an airstone and It's around 26-27 °C . I change the water once a week (conditioned water)- (2 litres or so). Also I have a couple of plants, no other tank mates just really small snails that came out of the plants when...

So I posted here a few days ago seeking advice about my betta, Clark, and never received any kind of response, but he's worse now! Two weeks ago he started swelling up on his left side, and a couple of days ago I started seeing white fluff and string-like things in his poo, so I suspected...